This article is a preview of the Spring 2016 issue of The American Prospect magazine. Subscribe here.

Now that Bernie Sanders has lost most of the once-industrial Midwest to Hillary Clinton, now that it's vanishingly likely that he'll become the Democratic nominee, the most important period of the Sanders insurgency has finally begun. The senator from Vermont has astonished both his fiercest critics and his (relatively few) longtime fellow socialists by mobilizing millions of voters, becoming a hero to the young, and being on track, by the time this year's primaries are done, to capture roughly 40 percent of the Democratic vote-all while running as a democratic socialist and scourge of Wall Street in this most capitalist of countries.

But Sanders's is not a campaign that history will judge by the number of votes he won. Like only a handful of predecessor campaigns, like no presidential campaign since Barry Goldwater's, his will be judged by whether it sparked a movement that transformed America. That's the metric by which Sanders himself measures his success: Whether his campaign can build what he calls a revolution, inspiring his supporters (and some of Hillary Clinton's, too), once this year's campaign is done, to build the political power and social movements that can break the hold that wealth exerts on politics and policy, and thereby re-create the mass prosperity that was once America's calling card to the world.

Problem is, electoral campaigns don't create enduring organizations, much less social movements. Though Barack Obama's 2008 presidential campaign likely mobilized more volunteers and donors than any that came before, the organization through which it sought to keep its activists active once Obama became president-Obama for America-lacked all autonomy or organizational life; it failed even to exert any pressure on Democratic members of Congress who were cool to Obama's agenda. Democracy for America, which sprang from the wreckage of Howard Dean's 2004 presidential bid, has never been more than a liberal pressure group of modest scale. Out of the more ideologically defined presidential campaigns of Jesse Jackson in the 1980s, the Rainbow Coalition emerged, but the Rainbow's goals were so consistently subordinated to Jackson's own political needs that progressive activists soon abandoned it.

This spring, however, leaders and activists from all manner of progressive movements and organizations are rolling this stone up the hill one more time. They can recite all the reasons why Obama for America never got off the ground; some of them even worked for the Rainbow until they realized there were better places to make social change. Most of them are painfully familiar with the tragic-comic history of the American left, a largely marginal tendency in American politics that has often squandered its moments of opportunity with displays of purity and rigidity that have only left it more marginal.

And yet, many progressives believe this time may be different. It's not that the Sanders campaign itself has incubated some kind of permanent left formation. "Bernie hasn't built an organization; he's built a campaign," says one left leader. "That isn't something that endures." The task of building that enduring something, they understand, falls to them-though Sanders himself can help them along.

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Leaders of unions, community-organizing groups, minority organizations and student groups, prominent environmentalists and Sanders activists, precinct walkers and online campaigners-some longtime allies, some total strangers to one another-are "all in one large, shifting conversation," in the words of one such leader, to figure out how to build the Revolution once the Sanders campaign is done.

Some are planning national conclaves, like the "People's Summit" in Chicago in mid-June, where the disparate groups in the Sanders universe will gather to lay out a common agenda. Some are planning how to prod the delegates at the Democratic Convention (including some pledged to Clinton) to shift the party well to the left. More fundamentally, they are debating ideas on how to create something-organizations, coalitions, networks, local, state, national-that can capture and build on the energy and politics that the Sanders campaign has unleashed.

The challenge of creating an enduring left out of Sanders's young supporters, who have brought the passion, energy, and numbers to his campaign, is particularly daunting.

"Presidential elections generate excitement unlike any other," says a veteran union leader. "They ignite a level of energy and self-activity that's hard to capture and transfer. We can't assume that 100,000 young people who have self-organized in the campaign are going to respond to being told, 'Here's the next big thing.' They won't come over if it's presented that way."

Will they come over at all? Are all these experienced activists even right in hoping that this time will be different, that this time a powerful social democratic left might just take root in America's political soil?

I think they are. Chiefly because Bernie Sanders's campaign didn't create a new American left. It revealed it.

IN 1906, GERMAN SOCIOLOGIST Werner Sombart wrote an essay entitled "Why Is There No Socialism in the United States?" Sombart was just the first of numerous commentators-among them Daniel Bell and Seymour Martin Lipset-who sought to explain why the United States, alone among industrialized democracies, never developed a major socialist movement. (Sombart's answer to this conundrum was that the upward mobility and higher living standards that European immigrants found here meant that socialism in America was stillborn.)

In the wake of the Sanders campaign, however, we suddenly need to pose quite a different question: Why are there socialists in the United States? Who are all these people who now not only flock to Bernie's banner but deem themselves socialists? What do they even mean by socialism?

The numbers astonish. In a Des Moines Register poll on the eve of the Iowa caucuses, 43 percent of likely Democratic caucus attendees said they were socialists. In a Boston Globe poll on the eve of the New Hampshire primary, 31 percent of New Hampshire Democratic voters called themselves socialists; among voters under 35, just over half did. And in late February, a Bloomberg poll of likely voters in the Democratic primary in South Carolina-South Carolina!-showed that 39 percent described themselves as socialists.

Favorable views of socialism aren't limited to Sanders supporters. The 39 percent of South Carolina Democrats who call themselves socialists exceeded by 13 percentage points the number who actually voted for Sanders. In a New York Times poll last November, 56 percent of Democrats-including 52 percent of Hillary Clinton supporters-said they held a favorable view of socialism.

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Nor was this sway toward socialism triggered by Sanders's candidacy. In 2012, a Gallup Poll showed that 53 percent of Democrats had a positive image of socialism, as did 62 percent of liberals. One year earlier, a Pew poll revealed that fully 49 percent of Americans (not just Democrats) under 30 had a positive view of socialism, while just 47 percent had a favorable opinion of capitalism. In 2011, the percentage of Americans under 30 who could have picked Sanders out of a police lineup was probably in the low single digits.

This is something new under the political sun. At no time in U.S. history have so many Americans supported a socialist presidential candidate, much less called themselves socialists. The apogee of socialists' electoral performance came in 1912, when Eugene V. Debs won 6 percent of the vote running for president on the Socialist Party ticket. What's more, the mystery of this socialist emergence is deepened by the fact that there is no visible organization in the United States that is recruiting people to socialism. The Democratic Socialists of America (of which I'm a vice-chair) has just several thousand members, and is almost entirely absent from many American cities. At first glance, this new socialist presence just seems to have sprung up, unsummoned, unannounced.

AP Photo/Jim Cole

And yet, it clearly has been building for years. Its emergence was foretold by Occupy Wall Street, and the polls that showed most Americans looked positively upon its message-that the 1 percent has flourished at the expense of the 99 percent-if not on the protesters themselves. It was foretold by the surprising rise to bestseller status of Thomas Piketty's Capital in the Twenty-First Century, by the success of the Fight for 15 movement in prompting cities and states to raise the minimum wage, and by two movements (in themselves, non-socialist, but nonetheless radicalizing) of the minority young: the Dreamers, demanding citizenship for undocumented immigrants, and Black Lives Matter, demanding an end to discriminatory criminal justice. More broadly, it was foretold by the rise of a distinct civic left: With millennials and minorities reshaping urban America, 27 of the nation's 30 largest cities now have Democratic mayors-the greatest urban partisan imbalance in the nation's history. Many of those cities have enacted groundbreaking progressive legislation-instituting and raising the minimum wage, mandating paid sick days, forbidding their police forces from cooperating with federal immigration authorities, giving collective-bargaining rights to independent contractors.

What's the substance of the new American socialism? I know of no surveys asking this newly hatched brood to define what they mean when they call themselves socialists, but we can make some educated guesses. First and foremost, they don't counterpose socialism to a militant liberalism. Indeed, the rising number of people who identify as socialists coincides with a rise in the number who call themselves liberals. Whereas in 2000, only 27 percent of Democrats told Pew they were liberal, by 2015 that figure had risen to 42 percent, and among millennials it had increased from 37 percent in 2004 to 49 percent today. In Bloomberg's poll of South Carolina Democrats, while 39 percent described themselves as socialist, 74 percent called themselves progressive, and 68 percent liberal: They weren't asked to pick just one.

That suggests one key to Americans' embrace of socialism: They've not been asked to choose among left-of-center political identities. By running as a Democrat rather than as a third-party alternative, Sanders has made it possible for progressives to back socialists and to call themselves socialist without worrying that they're voting for a Naderesque spoiler or marginalizing themselves from political life. As well, it's likely that when Americans call themselves socialist, they mainly have in mind the social democratic policies-a decent welfare state, more power for workers, and diminished devotion to the gods of the market-of Western European nations.

While this mass self-identification as socialists is new, the substantive conflation of social democracy with American liberalism is not.

In his 1972 book Socialism, Michael Harrington, the brilliant American socialist leader, called the American labor movement "an invisible social democracy"-the functional equivalent of openly social democratic Europe, sharing many of the same beliefs and goals (once the European social democratic parties had abandoned their commitment to nationalizing the means of production). To be sure, the U.S. unions were constrained to advancing social democratic policies within the Democratic Party, and only occasionally did they prevail. And it was precisely those occasions that were viewed as the high points, not of American socialism, but of American liberalism: Social Security and Medicare, and the Wagner Act, which legalized collective bargaining.

For its part, the American right always alleged that these were really socialist programs-and last November, Bernie Sanders said that the right was right. In a speech at Georgetown University in which he offered his definition of socialism, Sanders said it was precisely those policies, advanced by two liberal presidents, Franklin Roosevelt and Lyndon Johnson, which illustrated his vision of socialism. Indeed, there's little in Sanders's own program that hasn't been supported by many liberals who aren't Sanders supporters. While only six Democratic House members have endorsed Sanders, more than 60 favor single-payer health insurance.

Still, Americans on the left have almost always overwhelmingly preferred the liberal to the socialist label. Why, then, this sudden shift? One reason is the collapse of Soviet communism, that ferocious pretender to the socialist throne, has allowed younger Americans to identify socialism with the social democratic policies of Western Europe.

But the prime mover of millions of Americans into the socialist column has been the near complete dysfunctionality of contemporary American capitalism as it affects all but the top. Where once the regulated, unionized, and semi-socialized capitalism of the mid-20th century produced a vibrant middle-class majority, the deregulated, deunionized, and financialized capitalism of the past 35 years has produced record levels of inequality, insecurity, a shrinking middle class, and scant economic opportunities (along with record economic burdens) for the young. A recent study published in The Guardian revealed that the share of millennials describing themselves as middle class has fallen steadily since the turn of the century: from 45.6 percent in 2002 to a record low 34.8 percent in 2014, when 56.5 percent said they were working class, and 8 percent lower class.

AP Photo/Tina Fineberg

Therein lies what's new: The young women who are backing Sanders, for instance, are probably as feminist as their pro-Clinton elders, but their daily grievances against capitalism are as deep as those they hold against patriarchy, unlike many of their elders. In earlier times, many who backed programs such as those Sanders champions identified as liberal; but today, by calling yourself a socialist, you signal a break with and critique of an economic and political order that is rigged against you.

"On the reefs of roast beef and apple pie," Werner Sombart wrote in 1906, all socialist utopias run aground. To the immigrants who formed America's industrial working class, he argued, the living standards they found here so exceeded those they had left behind that going socialist became unnecessary. There are many other reasons why a mass socialist movement never came to America, but if, as Sombart contended, the reality and expectation of rising economic conditions, and the sense that this was a nation that rewarded work, was the key to socialism's absence, then the reality and expectation of today's declining economic conditions, and the sense that this is a nation that rewards only the rich, is the key to socialism's-or, more accurately, socialists'-surprising presence.

In 1967, as the ranks of the anti-war movement swelled both within and without the Democratic Party, a liberal activist named Allard Lowenstein took as his mission finding a prominent Democrat who would challenge Lyndon Johnson for the presidency the following year. Robert Kennedy kept putting him off, so eventually Lowenstein persuaded Minnesota Senator Eugene McCarthy to run.

If there's a Lowenstein in the Sanders saga, it's two left activists, Charles Lenchner and Winnie Wong. Both were in Zuccotti Park on the first day Occupy Wall Street was formed. Both were digital activists-Lenchner putting his digital-organizing chops at the service of unions and other progressive groups, Wong using hers to build the Occupy movement. With a group of Occupy alumni, Lenchner founded Ready for Warren, building a nationwide organization to persuade Massachusetts Senator Elizabeth Warren to run for president. "We wanted an alternative candidate even if we didn't yet have a candidate who wanted to run," Lenchner says, echoing a sentiment that Lowenstein once could have voiced. "It became a real force, but Ready for Warren didn't entice her."

Sanders, however, was interested. Lenchner and Wong transferred their energy to preparing a digital platform, People For Bernie Sanders, independent of Sanders's campaign, through which progressives could organize their own pro-Sanders activities. "We launched the day he declared," Lenchner says. Wong came up with a hashtag: #FeelTheBern.

They were not the only self-activating Sanderistas.

The Sanders campaign's decision to focus its early organizing on the first four caucus and primary states left his supporters in other states to find their own way to help his candidacy.

In Chicago, a progressive municipal coalition that had been organizing against police violence and for racial and economic justice issues, Reclaim Chicago, hosted the city's Sanders debate parties. In Seattle, says longtime labor activist Paul Bigman, the tens of thousands who flocked to a Sanders rally last fall were "not springing up from nowhere. There are a lot of strong movements here which naturally gravitated to Bernie, which we saw back in 1999 at the WTO demonstrations. The movement was already here." It simply hadn't had a presidential candidate to call its own.

Just as the anti-war movement preceded and shaped the insurgent presidential candidacies of 1968 and 1972, so a diverse movement for a range of progressive causes-economic justice most centrally-preceded and shaped Bernie Sanders's campaign. "This was somebody's base to have and to grow, from Occupy to Ready for Warren to Bernie," says George Goehl, the director of National People's Action, a nationwide organization of local working-class groups, with which Reclaim Chicago is affiliated. "Now Bernie's taken it to a whole new level."

The trajectory of the young people who came out of the anti-war movement and worked for the presidential candidacies of McCarthy, Kennedy, and George McGovern offers one template for where Sanders's supporters might end up. Many stayed active in Democratic politics, spearheading changes in party rules (most importantly, requiring delegates to be elected in primaries or caucuses rather than appointed by party bosses), forming organizations that favored a less militaristic foreign policy in the wake of Vietnam, winning over the Democratic Party to their position (over the opposition of the more hardline cold warriors in the party's ranks), and in time taking over much of the Democratic Party themselves.

It requires no imaginative leap to see the Sanders generation taking a similar course-fighting to change Democratic Party rules and positions, working to marginalize the sway that Wall Street has held in party councils. Larry Cohen, the former president of the Communications Workers of America who founded Labor for Bernie, is building support among delegates to this summer's convention for a resolution that would condemn the practice of Democratic candidates taking super PAC funds in future party primaries or caucuses. "Philadelphia has to make clear that this is now a populist party, not a finance-led party," Cohen says.

AP Photo/Jim Cole

But the forces that Sanders's candidacy has nourished face a more fundamental challenge than the anti-war young of the 1960s confronted: transforming not just a party or a foreign policy, but the economic and political order of the past four decades. The Sanders campaign has called the young to the barricades, but what will they do when it ends? "All these people who are starting to question capitalism and the role of the superrich," wonders Stephen Lerner, who led the campaign that successfully organized thousands of big-city janitors in the 1990s, "how do they dig into campaigns that substantively address those problems?"

Some in the Sanders generation will surely turn to electoral politics. "Environmental justice activists will run candidates for city council," Wong predicts. "Housing activists will do the same; so will racial justice activists. That's all going to happen in the next four years. This is a guarantee."

Can they count on Sanders supporters to help them into office? Sanders's campaign has amassed a list of millions of donors and volunteers. Yet candidates almost invariably husband their lists. Characteristically, they refuse to share them when their campaigns are over and tightly control the uses to which their lists are put. Given Sanders's commitment to a revolution, and given the likelihood that he'll not run a national campaign again, the hope throughout progressive circles is that this will be yet another convention that Sanders will shatter.

"The list is on everybody's mind," says one leader of an organization that's endorsed Sanders. "There's not a great history of this sort of thing working out. We're all curious to see how willing Bernie will be to say that there are a range of vehicles, groups, and campaigns that will carry on the revolution, or whether he will just form his own thing," which would monopolize the list.

"If all we end up with when the campaign is over is Bernie for America," says a leader of another pro-Sanders group, "I'll shoot myself."

THE GROUP THAT'S HAD THE MOST SUCCESS in building a vibrant left electoral force is the Working Families Party, which recruits, trains, and runs campaigns for hundreds of progressive candidates at the state and local level. Its name notwithstanding, the WFP-backed candidates generally run in Democratic primaries, frequently against more centrist opponents. While well established in New York, where it dominates New York City government, the WFP is only active in ten states. National People's Action, another group that mobilizes working-class voters for progressive candidates and causes, also has a presence in multiple states, though it has yet to develop a WFP-level of electoral expertise.

"Networks like the Working Families Party and People's Action can be a home for some of the folks coming out of the Sanders campaign," says Dan McGrath of Take Action Minnesota, a People's Action affiliate that has waged successful local and statewide electoral campaigns. "But they are not all going to move to one place. It will require state-by-state negotiations to capture what Bernie has built."

Some of the most vibrant and important organizations on the left today-Black Lives Matter, the Dream Defenders-either don't have a history of electoral involvement or see their work as entirely separate from electoral activity. Stephen Lerner, whose organizing preferences run to the non-electoral, believes that Sanders's electoral activists may nonetheless find some direct-action campaigns that address their particular needs. "The young Bernie supporters could take all the skills they used in the campaign," he posits, "to build a list of five million student debt holders who would demand to bargain with the Department of Education and the banks over the debt or else refuse to pay."

The leaders and activists of People For Bernie have no trouble, obviously, with electoral activity, but aren't that keen on traditional organizational forms. "Nobody can centralize the energy" that young people have exhibited on Sanders's behalf, says Wong. Adds Lenchner, her colleague, "These people's inclination is not to ask whom should I join, but what should my friends and I do? There's a move away from formal structures; it's a marriage between technology and the needs of this generation."

"There is a political culture clash," says one labor leader who's worked closely with the People For Bernie activists. "They would open-source this whole project and have individual activists do what they wanted. We believe in everyone marching together. It's like the anarchists meet the Stalinists."

"Political change happens slowly until it doesn't," says Working Families Party National Director Dan Cantor. "Bernie has changed what it's possible to say."

It's precisely because the limits of the possible have so suddenly expanded, in ways that make possible the construction of a genuine democratic left, that the discussions on how to build that left have become so intense. At a moment when self-proclaimed socialists are suddenly to be found on street corners-not, as in olden days, on soap boxes, but simply on the corners, likely fiddling with their phones-they need to find ways to come together, to find their voice, to build a force, or forces, for a more egalitarian America.

"We're one of many groups trying to figure out if there's some way to extend at least some of this energy sparked by Sanders into enthusiasm for down-ballot progressive candidates and issue campaigns," says Cantor. "We have to get the balance right between the energy of a swell of volunteer activity and the need to do long-term planning to mount a successful campaign. We don't have to figure out everything to figure out something."

Cantor has a clear sense of the challenge, at once electoral and doctrinal. "What we have to do is create a program that shows the contrast between what most working people want out of their government and what more corporate-minded Democratic members of Congress are willing to do," he says. "It's not conceptually complicated, but it's a lot of work."

That challenge would grow steeper if the most left-wing (or just most intransigent) Sanders supporters declined to support Hillary Clinton in a general election against Donald Trump or Ted Cruz, thereby estranging the vast majority of progressive institutions and individuals. It's inconceivable that Sanders himself or any of the traditional organizations that have backed his campaign would take that position, but some Sanders supporters have argued that Clinton is no better than the neo-fascist or extreme right-winger against whom she'll likely face off. (The one historic antecedent for such lunacy is that of the German communists of 1932 and 1933, who argued that their rival left party, the Social Democrats, were a greater danger than the Nazis. That they were proven wrong was small consolation.) True to its open-source principles, Wong says, People For Bernie won't endorse a candidate, but she makes clear that the group plans to "release explainers" on the candidates and how close the race is in various states. "We'll say, 'Here's why this is important if you live in a swing state.' We'll be responsible for, and to, the constituency we've created."

As polling makes clear, the leftward movement of the Democratic Party is not confined to Sanders supporters. When Democrats gather in Philadelphia in July, Sanders delegates are likely to put forth a range of proposals-Cohen's resolution that could take big money out of Democratic primaries, platform planks to further clamp down on Wall Street and oppose corporate free trade-that may well win the support of Clinton delegates and perhaps the nominee herself.

Al Drago/CQ Roll Call/AP Images

Whatever the organizational forms they may take, the Sanders forces will surely play a role that's critical-in both senses of the word-to a Clinton presidency. Sanders himself and Elizabeth Warren will doubtless endeavor to ensure that Clinton's economic policies aren't formulated by or infused by the spirit of the usual suspects from Goldman Sachs. The Sanders left will stomp on anything resembling the kind of grand compromise that cuts into such core commitments as Social Security, should Clinton seek the same kind of across-the-aisle deal that Obama did in 2011. Their own solution to breaking congressional gridlock, Cantor argues, should be waging campaigns to win back purple House districts from the Republicans, not with centrist Blue Dogs but with economic populists-a task that will require much on-the-ground organizing by Bernie's legions.

Should Trump or Cruz end up in the White House, the entire Democratic Party will shift to a militant oppositionist mode-a form of politics at which Sanders supporters, more than any other quadrant of the party, excel.

Economic upheaval has redefined the Democratic Party before. In the 1920s-like the 1990s, a time when business dominated policy and regulation was shunned-the Democrats' national chairman was John J. Raskob, a financial lieutenant of the Du Pont family, whose ownership of General Motors he helped secure. In the 1930s, as general strikes and an emboldened left pushed Franklin Roosevelt to enact groundbreaking economic reforms, Raskob helped found and fund the Liberty League, which opposed Roosevelt's re-election.

If the Sanders revolution is going to roll on, it must begin with a kindred redefinition of the Democratic Party-likely estranging in the process such Wall Street figures as Robert Rubin, whose deregulatory, pro-corporate preferences dominated Democratic policy in the 1990s just as Raskob's did in the 1920s. Indeed, when the Democrats convene in Philadelphia, the Sanders forces are likely to proclaim that theirs is just the latest in the occasional revolutions that have propelled the nation and the party forward, including the one that the Democrats held in Philadelphia 80 years before. In accepting the party's 1936 nomination for his second term as president, Franklin Roosevelt made clear how far the nation and the party had come from its corporate conservatism of the previous decade. Terming financial and corporate leaders "the privileged princes of these new economic dynasties, thirsty for power," he warned that they "sought control over Government itself," that "the political equality we once had won" was becoming "meaningless in the face of economic inequality," and that the preservation of "American institutions requires the overthrow of this kind of power."

If the Sanders generation can speak to America as boldly as Roosevelt did, and build their power once Bernie's campaign is done, they may just make their revolution yet.

This article has been updated.