CEDAR RAPIDS, Iowa (AP) — When Bernie Sanders addresses throngs of supporters who gather at his rallies, the divisions that plague the Democratic Party can feel far away. The Vermont senator speaks of building a “mutliracial, mutli-generational movement” that will cut through economic divides, catapult him into the White House and transform the nation.

Some of the highest-profile surrogates campaigning on his behalf are less sanguine.

Speaking at a concert for Sanders on Friday night, Rep. Rashida Tlaib, D-Mich., led sustained booing from the stage at the mention of Hillary Clinton, his rival in the 2016 primary. Rep. Alexandria Ocasio-Cortez, a New York Democrat who has campaigned for Sanders across Iowa, says the Democratic establishment should conform to the progressive movement, not the other way around. “We aren’t pushing the party left, we are bringing the party home,” she says.

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Then there’s filmmaker Michael Moore, who fires up Sanders crowds by bashing “corporate Democrats” and suggesting that the party’s own leadership may swoop in and steal the 2020 nomination from Sanders in a way that some of the senator’s supporters believe it did in 2016.

Such episodes demonstrate the tension at the heart of Sanders’ campaign as he shows signs of strength heading into Monday’s caucuses. While the self-described democratic socialist has never backed away from his call for political revolution, the visions of unity he also articulates are sometimes at odds with the rhetoric espoused by his supporters. The dynamic is playing out at a precarious time for the Democratic Party, which will have to unite to unseat President Donald Trump.

“The Sanders supporters are demanding that everybody unite behind Bernie, but if they want Democrats to unite behind Bernie they have to be ready to unite behind the moderate Democrats,” said Democratic strategist Brad Bannon. “And they’ve not yet shown that they will do that. They’ve not shown that, if things don’t go their way, they won’t just stay home in November.”

Sanders is bunched near the top of many polls in Iowa with progressive rival Elizabeth Warren and with former Vice President Joe Biden and Pete Buttigieg, the former Mayor of South Bend, Indiana, who represent the moderate wing of the party, along with Minnesota Sen. Amy Klobuchar. If he were to win the caucuses and also notch a victory in New Hampshire, which holds its primary on Feb. 11, Sanders will face growing pressure to show his campaign will be open to all factions of the party.

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Conversely, a series of losses would amplify calls on Sanders to ensure that his supporters rally behind the ultimate nominee. He insisted on Saturday that he would do just that.

“Let me say this so there’s no misunderstanding,” he told a rally in Indianola, Iowa. “If we do not win, we will support the winner and I know that every other candidate will do the same.”

Sanders has earnestly tried to quell intra-party division in other ways, too, describing many of his fellow Democratic presidential rivals as his longtime friends who are “good people.” But, often in the same breath, he gleefully fans the flames, calling his campaign the political and corporate establishment’s “worst nightmare.”

Sanders’ problem is he may only be able to achieve true unity by compromising on what many supporters see as his greatest strength: consistency over his decades in political office — even on positions that bucked this own party.

“For young people in particular, there’s an authenticity and a level of trust that is hard to garner from some of the other candidates,” said Evan Weber, political director for the Sunrise Movement, a youth-led activist group supporting the sweeping “Green New Deal” to combat climate change which has endorsed Sanders’ presidential bid. “His record is being consistent and relentless in demanding what he thought was just and was right for decades.”

But what some see as unwavering commitment to core ideals, others see as hostile.

“I just think he’s too angry,” said Paula Peeper, a 76-year-old retired office worker from Waterloo, Iowa, “especially when he says he’s the one to unite the party.”

Peeper, attending a rally Saturday for Buttigieg, said Sanders risks alienating voters in the closing stretch, especially when they see him leading in some Iowa polls, giving undecided voters reason to think harder about his rivals.

“It’s not helpful for Bernie to be fighting,” she said. “I think Biden, Pete and Klobuchar could be the beneficiaries of it.”

Melissa Dunlevy, 34, was a stalwart Sanders supporter and campaign volunteer in 2016, but now plans to support Buttigieg, thinking he could do a better job attracting Republicans and independents needed to beat Trump.

“I’m passionate about every single thing Bernie says, I’m 100 percent there,” Dunlevy said. “But it’s just another giant extreme, it’s another thing that’s so partisan, it’s another thing that divides us.”

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Associated Press writers Thomas Beaumont and Julie Pace in n Waterloo, Iowa, contributed to this report.