If you want to appreciate the return of the left as a significant force in American politics, treat yourself to a quiet evening with the 2012 Democratic Party platform. This vast accumulation of words, over 26,000 in all, contains no fewer than 42 invocations of "the middle class," 28 calls for "innovation," and 18 promises of "tax credits." Its first policy section places "Middle Class Tax Cuts" atop the list of Barack Obama’s achievements as president. And amid this great desert of focus-grouped language, boundless and bare, there rises not a single demand for a major universal public program.

How much has changed in the last few years. At the grass roots, the evidence for some kind of left-wing resurgence is too overwhelming for all but the most jaundiced or mechanical skeptic to deny: the wave of victorious labor strikes from West Virginia to Los Angeles, the advent of new activist movements like Black Lives Matter and the Women’s March, the rise of left political organizations (the Democratic Socialists of America grew from 6,500 members in 2014 to about 60,000 today), and the election of avowedly radical candidates to city governments, state legislatures, and Congress itself.

Amid this hive of activity, national opinion has pitched sharply to the left: Surveys now show record-high levels of support for same-sex marriage, legal marijuana, and a range of affirmative-action programs. Most striking of all, perhaps, is the apparent leftward shift in economic opinion. Recent polls have discovered commanding public support for social-democratic programs long exiled from mainstream politics: a national single-payer health-care plan, a federal jobs guarantee, universal parental leave and child care, and tuition-free college, along with heavy income and wealth taxes on the super-rich.

As the 2012 platform reminds us, it was not so long ago that Democratic Party leaders refused even to acknowledge the existence of such policies, let alone pay lip service to them. In 2013, President Barack Obama responded to the nascent Fight for $15 movement, which was beginning to organize fast-food workers, with a bold proposal to raise the minimum wage to $9 per hour. Throughout Obama’s second term, national politics churned on with barely a reference to a left-wing vision of the possible: In 2014, the phrase "Medicare for all" appeared in The New York Times just once, in a single forlorn letter to the editor.

Now the Democratic National Committee champions a $15 minimum wage on its website. More than 100 congressional Democrats—including presidential aspirants Elizabeth Warren, Kamala Harris, Cory Booker, and Kirsten Gillibrand—have lined up behind a national health-care bill co-sponsored by Bernie Sanders. The same presidential candidates have backed Alexandria Ocasio-Cortez’s call for a Green New Deal while touting a range of additional proposals like tuition-free college, a federal jobs guarantees, and direct cash transfers.

There is reason for skepticism about the sincerity and intensity of these new commitments, especially when brandished by ambitious politicians whose records do not match their rhetoric. But such weather vanes are finely attuned to the direction of the wind. And the atmosphere of center-left politics has changed since 2013, or even 2016, when Hillary Clinton chose to frame her campaign in Iowa around the certainty that single-payer health care would "never, ever come to pass."

What explains this swift ideological transformation? Over the longer term, no doubt, it owes its origins to the political and economic upheaval that has gripped much of the industrialized world in the last decade. Forty years of globalizing markets, deregulating states, declining unions, flattening wages, expanding debt, and skyrocketing inequality—administered by governments of the right and center-left alike—established the conditions for general political revolt, the reverberations of which could be seen from Wisconsin to Greece. Yet this broader crisis, by itself, hardly ensured a renaissance of social democracy in the United States. Across the Atlantic, the anti-establishment upheaval has boosted right-wing nationalists more often than movements of the left; its most immediate impact on American politics, after all, was the election of Donald Trump. And so one must look for more acute reasons for social democracy’s revival, and no better one can be found than Sanders’s 2016 campaign for president. So many of the ideas, tactics, and even key players of the current moment—including Ocasio-Cortez, a former Sanders-campaign volunteer—emerged in the context of that remarkable primary run. As new books by Sanders and his 2016 campaign manager, Jeff Weaver, help demonstrate, the Vermont socialist’s long-shot bid for the Democratic nomination did not just succeed in "pushing Clinton to the left"; it helped transform the shape and content of progressive politics in the United States.

Compared with the leading Democrats of his generation and of the generations that followed, Bernie Sanders lacks a certain courtly polish. He grew up in a rent-controlled three-and-a-half-room apartment in Brooklyn, where his father, like Warren’s father, worked as a salesman. But unlike Warren—or Obama, Bill and Hillary Clinton, Al Gore, John Kerry, Booker, Pete Buttigieg, and Beto O’Rourke—Sanders did not nurture his political ambitions on the campus of an Ivy League university. (If elected in 2020, he will be the first Democratic president since Jimmy Carter without a graduate degree.) At 38 years old—an age when Joe Biden was already fighting school integration in the Senate and Harris was enlisting millionaire donors to back her run for San Francisco district attorney—Sanders was working for the American People’s Historical Society of Burlington, Vermont, on a documentary about Eugene Debs.

If the social milieu of Sanders’s formative years was distinctive, his political education was even more so. At the University of Chicago, he joined the Young People’s Socialist League, read Marx and Lincoln and Dewey in the library basement, and fought for civil rights as a member of the Congress of Racial Equality. For the young Bernie, real politics was what happened outside the corridors of power: After being arrested at a Chicago sit-in, he told the writer Russell Banks, "I saw right then and there the difference between real life and the official version of life. And I knew I believed in one and didn’t believe any more in the other."

Real politics, for Sanders, also meant third-party politics. In the late 1960s, he moved to Vermont and spent nearly a decade running for state office with the left-wing Liberty Union Party. Though he never won much more than 6 percent of the vote statewide, he made headlines with his calls for worker-controlled businesses, publicly owned utilities, a guaranteed family income, and, at one point, a redistribution of the Rockefeller family fortune.

Such deep roots in third-party struggle make Sanders a black swan not only among today’s Democratic elite but across American political history. To find an influential national figure with such an extensive background outside the two-party system, you have to return to Debs and the Socialists in the early 20th century or, perhaps, Salmon Chase and the antislavery radicals who helped found the Republican Party before the Civil War. Like the political abolitionists of that era, Sanders has spent his life working to find a party to advance his cause, rather than finding a cause that can advance his party. Nor has that cause wavered very much in half a century. Interviewed by United Press International at the start of his first Liberty Union Party campaign in 1972, he produced a paragraph that could be pasted into a tweet today: "If we wanted to, we could have decent housing and free medical care and jobs for everyone…. It won’t happen because the wealth and money lies in the hands of a few people who are not concerned with the welfare of others."

In March 1981, Sanders ran as an independent candidate to become the mayor of Burlington. This time, he pulled off an astonishing upset, defeating the Democratic incumbent by a mere 10 votes and taking office just weeks after the inauguration of Ronald Reagan. Yet even as the international political climate turned against social democracy, Bernie forged ahead. Confronted with a uniformly hostile city establishment (Republican and Democratic alike), Mayor Sanders boosted worker-owned businesses and established a community land trust to fund affordable housing, the first such municipal program in the United States. Although the courts and the state legislature blocked his most ambitious efforts to overhaul the tax code, by the time he left office, Burlington had become a model city for egalitarian, public-led economic development.

Sanders was a pragmatic administrator of municipal government, but he never abandoned the politics of class struggle. Using his bully pulpit to defeat a waterfront development project that he called "a rich man’s paradise," he positioned himself as a champion of lower-income housing. In each of his three re-election campaigns, he won a larger share of the vote, driven by rising turnout from his young and heavily working-class base. And as he once again began to set his sights on winning a statewide seat, the song remained the same: His 2016 campaign manager, Jeff Weaver, recalls in How Bernie Won that during Sanders’s 1986 run for governor, the candidate delivered an identical lecture about "income inequality and tax fairness" in almost every town in Vermont. Sanders’s "message discipline and consistent use of the stump speech, while unexciting to the media, was a great strength…. It allowed him to carry out an intensely crammed schedule of events without having to incorporate new material."

This description by Weaver could serve as shorthand for Sanders’s entire career in politics. Journalists and academics worship at the shrine of originality, but for a social democrat in the late 20th century, consistency has proved the rarer virtue.

After two failed campaigns for state office, Sanders won a place in the House in 1990. ("I was never inside the Capitol until after I ran for Congress," he notes in his own new book, Where We Go From Here.) In Washington, he proved to be an effective legislator, passing more roll-call amendments in a Republican-controlled House than any other member. Real politics, it turned out, sometimes happened inside Congress, too, and Sanders fought in the trenches to eke out funding for low-income weatherization assistance and community health centers. In small but tangible ways, workers, consumers, and especially veterans benefited from his ability to cobble together unlikely voting coalitions on Capitol Hill.

In world-historical terms, though, Sanders swam against a heavy current. In August 1991, when the Soviet Union collapsed and capitalism’s tribunes proclaimed that history itself had come to an end, he was in Stockholm, on a trip to meet with Swedish Social Democrats and unionists. "Social democracy—a political party of the people—is something we want to take a good look at," he told reporters. But in Washington, the Democratic Party was looking harder than ever in the other direction, leaving Sanders to record protest votes against NAFTA, capital-gains tax cuts, and telecom and financial deregulation—all of them in bills supported by Bill Clinton, Joe Biden, and Nancy Pelosi. Watching the Democrats devote their 1996 national convention to the questions of "gun control, smoking, and the personal tragedy of a popular actor [Christopher Reeve]," Bernie fumed that there "was virtually no discussion of class, despite the fact that we have the most unequal distribution of wealth and income in the industrialized world."

Ideologically, Sanders was still nestled away in Burlington, a curious accident of the 1970s, compounding over the decades like a forgotten postal-bank account. His 2006 move from the House to the Senate did little to enhance his national influence, or to moderate his critique of the Democratic leadership. After the 2008 financial crisis, Sanders spent much of the Obama years waging a lonely campaign against the Wall Street–friendly status quo. One of just four members of the Democratic Caucus to vote against Timothy Geithner’s nomination as Treasury secretary, he also opposed Geithner’s replacement, former Citigroup executive Jack Lew, in 2013. "We need a Treasury secretary prepared to take on an oligarchy which now controls the economic and political life of this great nation," Sanders declared on the Senate floor. "Is Jack Lew that person? No, he is not." Was anyone listening? It’s not clear, since every Democratic senator, including Warren, voted to confirm Obama’s nominee.

In Washington, Sanders remained a figure of the fringe. When he announced that he would challenge Hillary Clinton for the 2016 Democratic nomination, pundits issued a collective guffaw. "Socialist to Snap at Clinton’s Heels," brayed The Hill. "There seems to be very little desire on the left for a challenger to Clinton," argued FiveThirtyEight, as usual better attuned to the Beltway elite than the electorate. But it was no coincidence that every loyal party progressive stayed out of the primary race: Obama and the Democratic leadership had worked hard to clear the field for Clinton, successfully discouraging runs by Biden and Warren, among other possible contenders.

This cleared field ironically produced the void that allowed an underdog campaign to flourish. Sanders made the most of his opportunity, balancing his attacks on the billionaire class with a bluntly social-democratic platform that promised health care, tuition-free college, and a living wage to every American. For the first time since Jesse Jackson’s 1988 campaign, the media were forced to cover a left-wing candidate seriously. And though much of this coverage was condescending, if not outright contemptuous, the message reached voters with remarkable speed. When Sanders entered the race in April 2015, he was polling at 5 percent nationwide; by August, he had a lead in New Hampshire. Democratic voters—and not just them, it turned out—were far hungrier for "political revolution" than anyone in Washington had guessed.

The stage was set for almost an entire year of fierce primary combat. For veterans of the 2016 campaign, Weaver’s How Bernie Won offers a chance to relive the glories and agonies of battle, complete with fulsome military analogies—one chapter is called "Stalingrad, Iowa"—and plenty of excerpts from the nonstop social-media crossfire. As Weaver notes, Clinton and Sanders fought each other on two different fronts and for two distinct prizes: the 2016 Democratic nomination, and the ideological trajectory of national politics thereafter.

On the first front, Sanders lost decisively. Although he won 22 states and more than 13 million votes, after the first few contests, he never threatened Clinton in the all-important delegate race. Weaver expends considerable energy lamenting the many reasons: the close collaboration between the front-runner and the Democratic National Committee; Clinton’s 20-fold advantage among superdelegates, which lent her an air of invincibility; and an unfriendly media eager to run hostile stories about Sanders (in the most egregious of many examples, MSNBC, The New York Times, and The Washington Post repeated and never corrected false reports of Sanders supporters "throwing chairs" during the Nevada state convention).

Yet looking back on the race three years later, the most important reason for Clinton’s primary victory was her overwhelming advantage in resources. Before a single ballot was cast, no fewer than 180 of 188 congressional Democrats had lined up behind her; eventually, 1,100 state legislators backed the front-runner, compared with just over 150 for Sanders. By July 2015, Clinton had already raised $46.7 million, plus $30 million in super-PAC money, far outstripping Sanders, whose small-dollar fund-raising operation was only starting to gain steam. When the race began, his projected campaign spending for all of 2015 was just $12 million.

The defining challenge for the Sanders campaign, as Weaver puts it in typically martial fashion, was "how a much smaller force in terms of resources and infrastructure would be able to defeat a much larger one." Sanders and Weaver made the only choice they thought they had, investing heavily in the first four state contests, Iowa and New Hampshire in particular. This strategy helped Bernie record the upsets that put him on the national map, but it left the campaign with few resources to compete in the pivotal Southern primaries immediately afterward. Clinton’s blowout victories in Texas, Virginia, Georgia, and Florida—where Sanders spent almost no money on direct media—gave her an insurmountable delegate lead and virtually ended the competitive struggle for the nomination.

Yet on the primary’s second major front—the battle for the ideological future of the country—Sanders has been an equally decisive winner. In the summer 2016 negotiations over the Democratic platform, Weaver recounts, Clinton’s team initially shot down proposals for a $15 minimum wage and universal tuition-free college while refusing to back away from Obama’s Trans-Pacific Partnership trade deal. Three years later, many Democrats now stand much closer to Bernie than Hillary on wages, college, and trade policy. The national debate on health care has shifted even more dramatically: "Medicare for All" has become a familiar and popular rallying cry, while Clinton’s opening position in 2016—that we must restrict ourselves to "improving" the Affordable Care Act with tax credits—is now almost starved of support outside of the insurance lobby itself.

It would be wrong to say that the Democratic Party, in its current institutional form, is anything like the party of Bernie Sanders. Business-friendly moderates still make up its largest congressional caucus, billionaire donors (and billionaire politicians) remain close to its leadership, and even many Washington progressives appear willing to dump Medicare for All for some plausible-sounding substitute. Nevertheless, the larger pattern is clear. On issue after issue, from wealth taxes to climate change to corporate PAC money, the national debate has moved away from the cautious incrementalism of the Clinton campaign and toward the bold social-democratic agenda laid out by Sanders in 2016.

For all his remarkable consistency, Sanders has evolved, too. Throughout the 1980s, he argued that the "corporate-controlled Republican and Democratic parties…are, in reality, one party—the party of the ruling class." His Vermont career was built not on cooperation with Democrats but on bitter competition with them. When he appeared at a county Democratic meeting to endorse Jackson in 1988, a local partisan slapped him in the face—for speaking against the official favorite, Massachusetts Governor Michael Dukakis, as well as for trying to break up the party by "passing himself [off] as a Democrat."

In the years since, Sanders has gotten much better at passing himself off as a Democrat—not in order to break up the party but to transform it through popular struggle. But as the 2020 primary takes shape, some have wondered if he will become a victim of his own success: With a slate of Democratic candidates who now endorse much of his 2016 platform, what distinguishes him from his progressive rivals?

On foreign policy, the gaps between Sanders and Democratic Party orthodoxy remain considerable. A reliable opponent of US defense spending and military interventions, from Vietnam to Libya, Sanders can boast an equally long record of skepticism toward America’s alliance with Saudi Arabia, which he spent three decades denouncing as a "feudalistic dictatorship." In the 2016 campaign, he largely steered clear of international affairs, but he has devoted much more attention to them since. He worked alongside Senator Chris Murphy to end US support for the Saudi war in Yemen; more generally, as he describes in Where We Go From Here, he has sought to connect his anti-interventionist instincts to a broader global confrontation with economic inequality and climate change.

Much about a possible Sanders foreign policy remains uncertain, including its approach to trade relations with China, its attitude toward Israel and Palestine, and its outlook on the 800-some military bases that the United States maintains around the world. Denouncing the "military-industrial complex" is one thing; dismantling it, quite another. Nevertheless, a view of international politics that rejects the premise of US global hegemony, as Sanders does, is already worlds apart from anything dreamed up within official Democratic circles, where enthusiasm for a new Cold War against Russian and Chinese "neo-authoritarianism" is brewing.

On domestic politics, too, Sanders remains distinctive. Since 2016, Democrats have remembered what Clinton’s campaign somehow contrived to forget: that rhetorical class war, including a defense of wage earners against the ultra-rich, is an effective electoral strategy. Yet the rest of the party envisions class politics primarily as a tool of partisan politics, a cudgel to bash Trump’s Republicans for their heartlessness and hypocrisy. Fundamentally, this is nothing new: The Democrats’ 2012 platform, for all its barrenness of vision, contained plenty of digs at the GOP as the party of "the wealthiest Americans."

Sanders, by contrast, understands that the rise of the 1 percent is a bipartisan phenomenon. "Our economy, our political life, and the media," he writes, "are largely controlled by a handful of billionaires and large corporations…. I believe that the Democratic Party bears an enormous amount of responsibility for this sad state of affairs." And unlike the party’s leadership, he recognizes that the power of the ownership class extends far beyond the nebulous special interests that every politician denounces. While no other national Democrat, Warren included, even uses the phrase "corporate media," Sanders rightly makes it a recurring theme in his critique of billionaire rule.

If this diagnosis of the problem diverges from the rest of the Democratic field’s, so too does his proposed remedy. A number of candidates now say they want to tame the ultra-rich with new taxes and regulations, while making essential goods more affordable for those earning lower incomes. But in contrast to means-tested programs like Harris’s LIFT Act, Sanders champions unapologetically universal programs: medical care, college education, living-wage jobs, retirement income, and environmental safety for everyone.

Contrary to much disingenuous criticism, this does not mean that Sanders refuses to recognize historically specific inequalities of gender and race. On questions of discrimination, pay equity, mass incarceration, and civil rights, his rhetoric and record have been far in advance of the Democratic Party’s mainstream for quite some time. (Many surveys have shown that Sanders’s strongest national support comes from women and people of color; the vast majority of black voters, regardless of their 2016 primary choice, continue to view him favorably.) The difference is not what Sanders does not say but what he does say: that every American, including those who voted for Trump, is entitled to the universal protections of social democracy.

Rather than battle Republicans by targeting a "Democratic base" defined by strict demographics—the preferred strategy of many progressives today—Sanders seeks to overcome the power of the ultra-rich by rallying a much larger coalition of the working and middle classes. Bernie’s America is not divided between red and blue states or vicious and virtuous voters but between the rulers and the ruled. This may be one reason Sanders, alone among the politicians classed as "progressive," has remained popular with independent voters.

Even without a Hillary Clinton–like juggernaut in the race, Sanders faces a tough fight for the 2020 nomination. Despite his broad popularity, there is little evidence that he has made inroads with his most difficult demographic, the very reliable voters over the age of 65. Nor is there much reason to believe that the politicians and donors atop the Democratic Party will view this Sanders campaign more favorably than the last one—and if an inconclusive primary race leads to a brokered convention, it is difficult to imagine him emerging as the nominee.

Above all, Sanders must contend with a mood inside the Democratic Party—powerful among its leaders and voters alike—that the only issue of consequence in 2020 is defeating Trump. Bernie’s struggle, from Chicago to Burlington to Capitol Hill, has always been much larger than defeating a single opponent. As he said in his March 2 speech in Brooklyn announcing his 2020 run, his goal is not simply to win an election but to build a movement that can "transform this country."

From Weaver to Ocasio-Cortez, nearly every progressive figure today is urging the Democrats to reclaim the bold mantle of Franklin Delano Roosevelt. Yet Sanders rounds out the introduction to Where We Go From Here with a quotation from another president who led an even bolder movement and whose election spurred an even greater transformation. The hoariest words in American history—Abraham Lincoln’s Gettysburg vow to defend "government of the people, by the people, and for the people"—are also, Sanders reminds us, some of the most radical. To overthrow an entrenched oligarchy and claim a "new birth of freedom" based on democratic equality for all: That would be a political revolution worth fighting for.