Warren’s message is aimed toward more traditional and mainstream Democrats who have fretted for months about the need to bring the party together; many believe President Trump won the election partly because the party was divided. Sanders’ pitch is pointed at young, infrequent and disillusioned caucus-goers who are exhausted and furious with the status quo, and who think Trump rode to victory on an anti-establishment mood throughout the nation.

“Warren’s plan seems to go hard on undecideds and bring them in,” said Rebecca Katz, a progressive consultant who advised Cynthia Nixon’s left-wing New York gubernatorial campaign in 2018. “Bernie is going for raw energy and enthusiasm and getting his people to caucus.”

Sen. Bernie Sanders. | Pablo Martinez Monsivais/AP Photo

To varying degrees, the left-wing rivals’ closing arguments in the first-in-the-nation state represent a departure from form. Sanders and his team have at times sought to make amends with the party leaders he clashed with in 2016, going so far as to ignore bombs thrown at him by establishment figures such as Clinton. Warren rarely talked about party unity until the last few weeks of the race.

But that all changed — and never was it clearer than this weekend. On Sunday, Sanders’ campaign posted a video on Twitter in which he said, “You can tell how good I feel by how nervous the establishment is getting.” He listed various groups that he believes are rattled — the Democratic establishment, Wall Street, drug companies, the fossil fuel industry, and Trump — because “we are their worst nightmare!”

Conversely, Warren’s TV and digital ads feature former Sanders and Clinton voters talking about why they support the Massachusetts senator, who’s been criticized for practicing a divisive brand of politics. “We can’t afford a fractured party in 2020,” one such spot begins with a former Sanders supporter speaking directly to camera.

The result of Iowa’s caucuses Monday and other early-state contests will shed light on whether Sanders’ or Warren’s approach has more sway in the progressive wing of the Democratic Party.

Filmmaker Michael Moore, campaigning for Sanders at a Bon Iver concert in Clive, said that “corporate Democrats” are “thoroughly pissed that Bernie Sanders is now number one, that Bernie Sanders might win this primary.” He also slammed the DNC for changing the debate rules to allow billionaire Michael Bloomberg on stage while not doing the same for Cory Booker or Julián Castro “‘cause he’s got a billion fucking dollars!" At the same show, Rep. Rashida Tlaib booed when a moderator began asking about Clinton saying “nobody likes” Sanders.

At a town hall in Indianola, Rep. Ilhan Omar, a major Sanders surrogate, said, “You may have noticed that the attacks on Bernie have become more and more desperate. You may hear a lot of labels being thrown out: ‘Bernie’s too radical, Bernie’s too dangerous, Bernie’s too polarizing.’” Omar said the same criticisms had been lobbed at Martin Luther King, Jr. and other multiracial, working-class movements. She also said “we are going to be sending Sen. Bernie Sanders to the White House” and “there are so many people” who are “freaking out about that prospect."

At a Warren rally on Saturday, Rod Sullivan, a Johnson County supervisor who was one of the first elected officials in Iowa to endorse Sanders in 2016, spoke, followed by Sue Dvorsky, the former chair of the Iowa Democratic Party who was a prominent Clinton backer in 2016. Dvorsky led the chant, "It's time! It's time! It's time for a woman in the White House!”

In a rare change to her stump speech, Warren spoke fondly of other Democratic candidates who have left the race, including Booker and Kamala Harris. “I’ve been building a campaign from the beginning that is not a campaign that’s narrow, not a campaign that says it’s us and nobody else,” she told voters in Cedar Rapids at Coe College. “It’s a campaign that says, ‘come on in’ because we are in this fight together.”

Sullivan said in an interview after the event that he wasn't sure if his pairing with Dvorsky was an intentional message, "but most things are pretty well thought out, so maybe."

“I honestly think Sen. Warren is the perfect candidate to bridge that gap that we have in 2016,” he said. “I really do.”

Warren’s chief strategist, Joe Rospars, tweeted Sunday that her yard signs have read “Vote Democratic” in the corner “since day one" — an implicit message that Warren is a party woman (and Sanders, who is still an independent, is not a party man).

Faiz Shakir, Sanders’ campaign manager, told POLITICO that the anti-establishment message is “core to the DNA of this campaign from the beginning.” But asked why it has been delivered more forcefully recently, he noted that attacks from traditional Democratic leaders have been escalating.

“Why now? Just let’s be honest about the facts first of all,” he said. “You have a lot of establishment forces who are expressing handwringing and concern about the Bernie rise.”

John Kerry, a top surrogate for Joe Biden, was among them. The party’s 2004 nominee said on Saturday that he doesn't know if Sanders is a Democrat and that “he’s been a socialist the entire time he’s been in the Senate." Clinton went on a podcast Friday to again accuse Sanders of failing to work to unite the party in 2016.

Shakir insisted that Sanders also advocates unity: “If you’re bringing people into the political process from different backgrounds, people who have never caucused before, first-time voters, young people, you are showing the kind of unity approach you would need to defeat Donald Trump.” Sanders regularly says in his stump speech that he wants to bring people of all stripes together and that Trump wants to “divide us up.”

For all the Sanders campaign’s anti-establishment fervor, some on his team did try to extend an olive branch to mainstream Democrats in the frenzied days before the caucuses. After Tlaib booed Clinton, Rep. Ro Khanna, Sanders’ campaign co-chair, said Clinton’s record of expanding health insurance for children and defending women’s rights should be respected.

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“We will bring progressive change through addition, not subtraction,” he told CNN. Shakir also said Tlaib’s booing was “a mistake,” and she “acknowledged it was a mistake.”

Nick Merrill, Clinton’s longtime spokesman, said: “Rep. Khanna is right: Elections are about addition, not subtraction, and there’s a pattern emerging here that plays directly to the electability question for Sen. Sanders and his campaign.”

Of Democrats concerned about Sanders’ ability to unify the party, Shakir said they miss the fact that the Vermont senator is part of the Senate Democrats’ leadership team and respected among other members of the chamber.

“Four years ago, maybe it was a little bit different. His run was seen as a little bit more concerning. And now this time around, you go around and talk to his colleagues, they see him as having been a value-add to this party, bringing people into it,” he said.