MASON CITY, Iowa—They say this Democratic candidate for president—the one running against Hillary Clinton—can’t possibly win a national election. But Susan Sarandon, the Oscar-winning liberal actress, was here to remind the people of this small Iowa town that they’d heard that line before.

“Last time, the people of Iowa didn’t listen to the machine,” Sarandon said, russet-colored hair framing her famous face as she looked out on the Music Man Square, an indoor fake streetscape commemorating the birthplace of the famous musical’s author. “They said he was unelectable—a black man with a funny last name. Well, here we are again, facing the machine.”

In Sarandon’s telling, the unkempt socialist senator from Vermont is the Barack Obama of 2016. To many Sanders supporters, Obama’s successes—from the Iowa caucuses to tough two national elections—render moot the argument Clinton is once again making that she’s the only one who can win.

But few would have expected the party to consider, for its post-Obama future, a candidate like Sanders, who is in many ways the opposite of Obama’s gauzy, transpartisan, future-oriented, multiracial appeal. Sanders is sharply ideological, beloved mostly by white liberals, and represents the party’s 1960s protest-movement past more than its techno-utopian future. The reason Sanders and Clinton have spent so much time bickering over which of them is the heir to Obama is that there is no Barack Obama in the Democrats’ 2016 primary. If there were, the party might not find itself so bitterly divided.

“A lot of Democrats doubt that he can be nominated and be successful,” Richard Schinnow, a 76-year-old retired community-college teacher in a Patagonia jacket and glasses, told me as we waited for Sanders to appear. “But I do think that’s changing.” Particularly with a field of Republican candidates that is “so weak and so weird,” he said, many Democrats believe the presidency is theirs no matter who they nominate—so why not go with their hearts?

Mason City, two hours north of Des Moines, has a population of less than 30,000, but the hall was packed to capacity, with more than 1,000 people sitting in chairs, standing next to the hall’s quaint storefronts (dry goods, menswear), and listening from the next room. They cheered Sarandon’s arguments and cheered louder when Sanders took the stage. It’s one thing to draw thousands of kids to a Vampire Weekend concert on a college campus, as Bernie would do a few days hence. It’s another to bring them out here.

Whether or not Sanders succeeds, his rise to near-parity with Clinton, supposedly his party’s most inevitable candidate in many years, vividly illustrates the central revelation of the 2016 election: the lurking anger and radicalism that now exists on both sides of the electorate. The 2016 campaign’s twin unexpected phenomena—the parallel rise of Sanders and Donald Trump, who currently command an eerily similar 37 and 36 percent of their parties’ national vote in poll averages—have taken by surprise the pundits and wise men and party establishments who fundamentally underestimated partisans’ desperate desire for extremes.

The Beltway looked at Sanders and saw a raging figure on the fringe—an anomaly. If Sanders pulls out a win on Monday, he will have proven yet again that what seemed far-out was actually the party mainstream.

Sanders’s supporters love to ridicule the notion that socialism is something scary. They embrace it—they’re glad he’s made it a word you can say in public again. “I’m also a socialist,” John Dallas, a goateed Mason City housepainter, told me matter-of-factly. “I mean, how’s that capitalism working out in the United States? It’s fine for the top one-tenth of one percent, but the other 90 percent of us aren’t doing so well.”

And so Sanders has exposed the left-wing ideals in the hearts of a substantial portion of Democrats, just as surely as Trump has illustrated liberals’ stereotypes about Republican nativism and xenophobia. A Democratic Party that strained for decades to position itself as moderate, and to ostracize its radical voices, has instead seen its long-suppressed liberal energy burst to the fore.

“I think he’s more radical than the other people we’ve had, and I like that about him,” Taylor Raska, a 28-year-old bartender with a nose ring, mismatched earrings, and lines of cursive writing tattooed on her arms, told me. An ardent environmentalist who’s tired of politicians, Raska believes the old system must be smashed for a new order to take its place. “Everything’s going to change!” she said, savoring the beautiful thought. “We are in this amazing period—it’s awesome to be a part of. Everything is changing!”

People who feel like they’re struggling against long odds are fed up with the solutions that have been tried. There was a plant in Mason City that made filters, but it went to Mexico some years ago, a 62-year-old named Sue McKee told me. She teaches the GED class that the laid-off men in their 40s, 50s, and 60s come to, desperate to work again and needing a high-school diploma for the first time in their lives. “We shouldn’t make it so hard for people,” she said. On Monday, she planned to register with the Democratic Party and caucus for the first time in her life.

When Sanders got up to speak, the crowd reacted ecstatically to his exhortation to  stage a “political revolution.” For more than an hour, he hunched over the lectern, gripping it with both hands and decrying the “rigged” economy. Sanders’s presentation is largely the same as it was six months ago, even as his campaign has professionalized and reoriented—his dark-blue backdrop featured a new slogan with obvious echoes, “A Future to Believe In.”

“Anybody here who thinks real change comes easy,” Sanders proclaimed, “knows nothing about American history.”

How severe are the Democrats’ divisions? The factions seem nowhere near as tribal and mutually incomprehensible as the Republican crackup; most Sanders supporters I met said they respected Clinton, even if they didn’t trust her, and Clinton supporters said they get where the Sanders people are coming from.

But in one respect, the Democratic revolt is worse, because the party didn’t see it coming. Republicans have known for years they had problems, a restive base they couldn’t control. Democrats are just now finding out.

It’s still a mystery why, exactly, Clinton lost control of this race. As of mid-December, she seemed to be sitting pretty, winning the debates, leading in Iowa, and catching up to Sanders in New Hampshire. But sometime around the new year that changed, and Sanders surged.

Was it Trump’s insults, Sanders’s ads, a fluke of the polls? Nobody could really say. But the professionals behind Team Clinton reacted to this surprise development with all the grace and equanimity of a mule that’s just been bitten by a deerfly and is thrashing wildly around. Clinton attacked Sanders with wild implausibility, insisting he was both too left-wing to win and not liberal enough on certain issues, primarily gun control. When Sanders lashed rather mildly back, her campaign flew off the handle, accusing him in dire terms of a viciousness ill-befitting his gentle reputation. Perhaps there are Iowa Democrats who were gravitating to Sanders until they heard he had “gone negative,” and stopped in their tracks; I have yet to meet them.

In a middle-school gym in Newton, half an hour east of Des Moines, the basketball scoreboard’s clock was cleverly set to 20:16, as it is at all Clinton speeches. Laura Engel, a 54-year-old who sells baked goods out of her home and volunteers with the local Democratic Party, was eagerly anticipating the woman she considered the “future president of the United States.” But Engel didn’t plan to help Clinton reach that goal herself.

“I’m going to caucus for Martin O’Malley,” she said. “I don’t think anybody is paying attention to him, and I think they should.” She admired Clinton’s qualifications and considered her nomination assured. “But I have so many female Democratic friends who won’t vote for her, and I worry about that,” she said. Sanders’s call for socialism was too far out; O’Malley seemed like the safe middle road.

You don’t meet a lot of undecided voters at Sanders’s events—at this point, most are committed. That’s not the case with Clinton, whose celebrity draws out people who aren’t sure about her, people who figure she’ll get the nomination no matter what they do, and even people like Engel who have made up their mind to support somebody else.

Most D.C. Democrats still do not believe Clinton to be in much real peril. On Saturday, she pulled narrowly ahead in the final Des Moines Register Iowa poll. Even if she loses Iowa and New Hampshire, the thinking goes, she’ll be saved by South Carolina, where her support among minority voters has given her a larger lead on her rival. (Bill Clinton lost both Iowa and New Hampshire in 1992.) Sanders, however, hopes an Iowa-New Hampshire one-two punch will set off a chain reaction, dominos falling unstoppably as Clinton relives her miserable experience of 2008.

The sensibility of the Clinton voter is recognizably distinct from Sanders’s fans: Clinton voters values moderation, temperance, and common sense, and can’t understand why everyone else seems to be yelling at each other. These are the voters disenfranchised by the current moment in our politics. They don’t want to smash the system, just to improve it. And nobody but Clinton seems to be speaking their language.

“I wish she would take ‘fighting’ out of her slogan—it’s too aggressive,” Jerry Manternach, a 73-year old retired prison counselor, told me, appraising Clinton’s backdrop—a giant blue “H” with the words “Fighting for Us” superimposed. “I’m not angry so much,” he added. “I wish politicians could come together and solve problems without fighting and being bellicose.”

Manternach and his wife, Carolyn, were probable—but not committed—Clinton supporters. They’d stopped by her rally because they were in the neighborhood. “Bernie, he’s just a cool guy,” Manternach told me. “I believe in what he believes in.” But in the end, he gave more weight to Clinton’s experience, he said.

This is Clinton’s challenge: to argue for an extension of the status quo at a time when few people are satisfied with the way things are. Even among Democrats, most of whom support Obama, that’s a tough sell. “There’s still more to be done, but we can build on what’s been accomplished,” Rachel Summers, a 46-year-old city worker, told me. “We don’t need to overthrow everything.”

Clinton, in her speech, cast herself as the candidate of dogged incrementalism and detailed, specific proposals. She pledged to take on the big banks, to bring back jobs, to raise people’s incomes but not their taxes. (In 2007, Newton’s Maytag washing-machine factory closed, taking with it 3,000 jobs.) She spoke briskly and cogently, in well-practiced paragraphs, and took several questions, standing before the audience with a microphone and no lectern. She cast Sanders’s grand plans for things like single-payer health care as destabilizing. She hugged a woman who’d lost her home.

“We have got to do more to help people,” Clinton said, as cameras clicked away, capturing the rare moment of intimacy. Later, she concluded, “Let’s move away from all this meanspiritedness, this insulting!”

In these unsettled times, Clinton’s stay-the-course appeal felt like a time capsule, a postcard from another era when sensible centrism was still the rule in politics. But we live in a time of revolution, unsettled and restive. Even if Clinton achieves the nomination that has eluded her for so long, this will be the lesson of Bernie Sanders’s rebellion.