On Monday, CNN hosted a live town hall forum for a single presidential candidate, Senator Bernie Sanders.

The second question from the audience summarized the Democratic front-runner’s predicament. “There’s a lot of misinformation regarding your plan for universal health care,” a woman began. “Would you please provide a brief overview of your plan and how it differs from that of other candidates?”

The Vermont senator outlined his proposal—the federal government covers all nonelective medical costs, basically—but then he left the rest of the question open for viewers to deduce the differences between his plan and that of fellow candidate Senator Kamala Harris (D-California). In the past couple of months, CNN has hosted three town halls starring Harris, Senator Amy Klobuchar (D-Minnesota), and former Starbucks chief executive Howard Schultz, who is exploring a potential run as an independent candidate. Harris, too, touted “Medicare for all” at her town hall four weeks earlier. Indeed, she and several other Democratic presidential candidates have cosponsored Medicare-for-all legislation authored by Sanders.

But Harris—facing conservative derision for her answer—quickly hedged her enthusiasm to “eliminate” the private insurance market. Less than a day later, she further amended her response to welcome “alternative,” “moderate” proposals short of Sanders’s vision for a single-payer health care system.

Sanders seeks to lead a party that can barely stand to mention his name and explicitly cite his influence. The independent senator who identifies as a Democratic socialist authored conventional wisdoms that have come to dominate modern Democratic politics, and now he confronts several presidential rivals who might mimic and displace him. “Before I ran for president in 2016,” Sanders said, “[Medicare for all] was considered to be a wild and crazy idea.” It’s not so prohibitively wild now. Though he remains indispensable, Sanders isn’t so alone.

The 2016 Democratic presidential primary was, supposedly, an exhausting and destructive process. A 14-month bloodbath. Hillary Clinton and Sanders, the front-runner and the insurgent, respectively, devoured one another. Clinton—arguably the second-most-divisive figure in modern U.S. politics—won the nomination but lost crucial confidence among her party’s younger, left-wing contingents. Sanders was a sore loser; Clinton was a hopeless winner. She limped into the larger confrontation with now-President Donald Trump and suffered from the contrast between her rigid, conventional choreography and Trump’s loud, counterfeit candor. Clinton won the popular vote but lost the Electoral College, having lost former president Barack Obama’s crucial contingents in Ohio, Michigan, and Wisconsin. Trump mimicked Sanders to undermine Clinton in these key states. “Every insider getting rich off of our broken system is throwing money at Hillary Clinton,” he argued in his stump speech. “The hedge fund managers, the Wall Street investors, the professional political class—it’s the powerful protecting the powerful. Insiders fighting for insiders. I am fighting for you.”

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For 14 months before Clinton won her party’s nomination, she and Sanders argued about capitalism. It was an unthinkable pretext for a party that fears comparisons to socialism, communism, and Europe in general. Al Gore and Bill Bradley were not about to spend the 2000 primary season grousing about wealth creation, nor was Howard Dean prepared to abandon the private health insurance market. Clinton mounted a tough-loving defense of free enterprise: “It is our job to rein in the excesses of capitalism so that it doesn’t run amok and cause the kinds of inequities that we are seeing in our economic system,” she argued in one televised debate. “But we would be making a grave mistake to turn our backs on what built the greatest middle class in the history of the world.”

Sanders doesn’t broadly condemn capitalism so much as he resists the easy valorization of wealth creation. His 2016 presidential campaign amounted to a cathartic, left-wing critique of the global economy, which had, for the past decade, shocked the national conscience. For once, Democrats argued openly—angrily—about private capital and public services. Sanders, the outsider, posed annoying questions about the Democratic Party’s role in the neoliberal project. Had liberal Democratic policies protected anyone? Could the party do better than middle-class tax cuts? Sanders forced pundits, activists, and voters to reevaluate the broader Democratic project. His campaign was a referendum on a party that had spent the past half-century rehearsing Republican style and priorities. Suddenly, Democratic politicians take social democracy seriously. So, too, do many voters.

Clinton has so far declined to launch a third presidential bid. The former secretary of state has lost twice already, largely due to her tragic misapprehension of Democratic politics in the new century. Two years after she won the national popular vote, her critics in both major parties have demoted her to resentful obsolescence. Clinton commands a large following, but she rarely, if ever, commands broader arguments about liberalism, the Democratic Party, and the future: As a candidate, Clinton has been a political movement unto herself. Meanwhile, Sanders has remade the party’s larger thinking without serving primarily as a Democrat.

Withstanding Clinton, Sanders faces his most formidable contrast in the form of Obama’s vice president, former Delaware Senator Joe Biden, the charming, moderate alternative to the left-wing rebellion embodied by Sanders and emulated by senators Elizabeth Warren, Cory Booker, Kirsten Gillibrand, et al. Klobuchar, who has struggled to build a campaign team due to reports about her mistreating Senate staff, has positioned herself as the most conventional, center-left figure in the current primary bunch, and even she is stumping for universal health care. “The Affordable Care Act is a beginning and not an end,” Klobuchar said in 2014. For many Democrats, Biden embodies an old-school political style in his friendly, paternal mannerisms and an old-school political liberalism in his resistance to comparisons with socialism that Sanders and his successors openly cultivate. Rather, Biden appeals to older, disenfranchised moderates through sheer force of his public persona. He’s folksy, but he’s no populist. He’s pugnacious, but he’s no revolutionary.

But revisionism and socialism aren’t really—fully—Biden’s problems. Biden wouldn’t be the only 2020 Democratic hopeful who would have to account for an inglorious legislative record; the leading Democrats, including Sanders, are all senators, after all. Biden’s distinct advantages seem self-evident: his stature, his gravitas, his favorability. What self-respecting folk archetype from Wisconsin wouldn’t want to vote for cheesy, dear old Joe Biden? Still, Biden’s own political constitution is a mystery. In half a century of public service, he hasn’t distinguished his outlook on Democratic politics. Biden’s most substantial leadership term—as Senate Judiciary chairman from 1987 through 1995—is now largely defined by his failure to investigate Anita Hill’s sexual harassment accounts about now–Supreme Court Justice Clarence Thomas. He’s run for president twice already—disastrously in 1988 and marginally in 2008—only to rank along the bottom and drop out before Super Tuesday. In 2016, Biden nearly ran against Clinton and Sanders, only to withhold himself from contention following the May 2015 death of his son Beau. He has agonized for the past several months about launching a 2020 presidential bid. The Democratic base may be starving for Biden’s viability against Trump, but it’s hardly suffering for a lack of Biden’s ideas.

Sanders’s ideas are something different. He isn’t a Democrat—and yet he’s radically improved the party’s discourse and responsiveness to its own political base. Sanders has rescued the party from its terminal sheepishness and has discouraged the party’s leading presidential candidates from the previously ceaseless urge to run to Biden’s right. Indeed, Sanders has emboldened his own competitors, who have appropriated enough of his priorities and posturing to risk rendering the 77-year-old senator obsolete. Already, he is struggling to address the gender and racial dynamics—often maligned as “identity politics”—of the most diverse presidential primary in U.S. history. “We have got to look at candidates not by the color of their skin, not by their sexual orientation or gender and not by their age,” Sanders told NPR a week ago. At Monday’s town hall, Sanders answered a pointed question about his prominence, and his credibility, in this presidential contest: “I am enormously proud of the fact that we have the most diverse, progressive freshman class in the history of the United States Congress,” Sanders said. “And you know what I did in 2016 and in 2018? I ran all over this country to try to make that happen.”

So Sanders supporters, and the Democratic Party in general, suffer the Sanders paradox. If he really is uniquely committed to social democracy in short order, then the Democratic Party is hopeless: Who will stump for social democracy once Sanders has left the scene? In the House, there’s freshman Representative Alexandria Ocasio-Cortez (D–New York)—and there’s only one other socialist in Congress quite like her.

Sanders led the Democratic Party further left, Trump led the Republican Party further right, and both men embodied a loosely defined new populism.

Given the benefit of Trump’s incumbency, Republicans are hardly so inclined to host a second existential crisis at the presidential level. Trump is running for reelection, and whatever ambivalence conservatives might have once broadly expressed, rather vividly and passionately, about the president last time, Trump is now the only game in town. In South Carolina, party officials may cancel the state’s February 2020 contest to spare Trump a primary challenge from center-right candidates such as former Ohio governor John Kasich and former Massachusetts governor Bill Weld. “Considering the fact that the entire party supports the president, we’ll end up doing what’s in the president’s best interest,” South Carolina GOP chairman Drew McKissick told the Washington Examiner. Meanwhile, Bill Kristol has piled Trump’s detractors into a single life raft bound for a luxurious exile. Self-appointed hitman Tucker Carlson is hunting Trump’s conservative critics—Mitt Romney, Kristol, Max Boot—for sport. The Republican Party is closing. For two decades, GOP leaders struggled, however half-assedly, to broaden conservatism’s appeal and thus expand their base. In 2020, the Republican establishment will double down on Trumpism—a right-wing movement defined by its most repulsive, exclusionary impulses.

In centrist theories and conservative assessments, Sanders risks alienating moderate voters, including the mythical Obama-Trump supporters who might otherwise break in favor of a more conventional Democrat in the 2020 general election. It’s the classic and potentially insurmountable fear about Sanders, whom would-be Biden supporters have already come to resent as vividly as the Clinton campaign did.

“Democrats netted 40 seats in the midterms and did so with a lot of candidates that pundits in D.C. would dismissively call ‘center left,’” one Biden confidant recently told The Atlantic. The 2020 Democratic primaries may indeed pit “liberals” against “leftists” in a grudge match that ends with party loyalists agreeing to split the difference in the form of Harris. Sanders may struggle to differentiate himself from Elizabeth Warren. But his first presidential campaign wasn’t entirely about left-wing notches on a simplified political spectrum. In 2016, Sanders didn’t campaign as a candidate more liberal than Clinton; he campaigned as a different kind of liberal—the rare, persuasive U.S. socialist. His supporters believe him to be one of a kind. One can only hope they’re wrong.