Sens. Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE (I-Vt.) and Elizabeth Warren Elizabeth WarrenWarren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon No new taxes for the ultra rich — fix bad tax policy instead MORE (D-Mass.) appear to be at war after a long-standing detente, unnerving liberals who hoped to see a progressive win the party’s nomination and fear the clash could hurt both presidential candidates.

Tensions between the Sanders and Warren camps were already rising, but they boiled over ahead of a pivotal Tuesday night debate in Des Moines, Iowa, after CNN reported that Sanders told Warren at a meeting in 2018 that a woman could not be elected president.

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Sanders vehemently denies the story, which came from four anonymous sources close to Warren.

Warren, in a statement released late Monday, said Sanders did make the remark, but she also sought to cool tensions.

“I thought a woman could win; he disagreed. I have no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences on punditry,” she said.

The back and forth comes on the heels of a weekend dust-up between the two candidates after a report that Sanders’s volunteers had been instructed to tell voters that Warren would be a weak general election candidate because she only appeals to wealthy, white liberals.

Those developments appear to spell the end of a long-standing truce between Sanders and Warren, who are running first and second in the latest Des Moines Register poll of Iowa with the caucuses only weeks away.

Liberal Democrats, some of whom would be happy with either Sanders or Warren winning the nomination, are fearful that the attacks between the two will pave the way for a centrist candidate, such as former Vice President Joe Biden Joe BidenCast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response Biden tells CNN town hall that he has benefited from white privilege MORE or former South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBogeymen of the far left deserve a place in any Biden administration Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq A socially and environmentally just way to fight climate change MORE, to win in Iowa.

The progressive group Democracy for America released a statement on Monday calling on Sanders and Warren to stand down.

“Bernie Sanders and Elizabeth Warren, you both are progressive champions and our movement needs to see you working together to defeat your corporate Democratic opponents – not attack each other,” the statement said. “Progressives will win in 2020, but only if we don’t let the corporate wing or [President] Trump divide us.”

The CNN report about Sanders’s alleged sexist remark was a dramatic escalation after a year of campaigning in which the candidates and their campaigns refused to go after one another.

Sanders released a statement strongly denying that he had ever told Warren that a woman could not be elected president. The Vermont senator, who has at times singled out CNN and other mainstream outlets over their coverage of his campaign, called the report “ludicrous.”

“It’s sad that, three weeks before the Iowa caucus and a year after that private conversation, staff who weren’t in the room are lying about what happened,” he said. “What I did say that night was that Donald Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE is a sexist, a racist and a liar who would weaponize whatever he could. Do I believe a woman can win in 2020? Of course! After all, Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE beat Donald Trump by 3 million votes in 2016.”

Sanders’s supporters are furious over what they view as a desperate attempt by the media and Warren staffers to blunt his momentum in the race.

Politico reported over the weekend that the Sanders campaign quietly instructed volunteers to tell undecided voters that Warren would be a weak general election candidate because her appeal is limited to “highly educated, more affluent people who are going to show up and vote Democratic no matter what.”

According to the talking points, the volunteers were instructed to tell voters that Warren is “bringing no new bases into the Democratic Party.”

Warren fired back directly at Sanders, saying she was “disappointed to hear that Bernie is sending his volunteers out to trash me.”

“Bernie knows me and has known me for a long time,” Warren said. “He knows who I am, where I come from, what I have worked on and fought for and the coalition and grassroots movement we are trying to build. Democrats want to win in 2020. We all saw the impact of the factionalism in 2016 and we can’t have a repeat of that.”

“I hope Bernie reconsiders and turns his campaign in a different direction,” Warren added.

Sanders sought to distance himself from the report.

“We have over 500 people on our campaign,” he said. “People do certain things. I’m sure that on Elizabeth’s campaign people do certain things as well.

“But you’ve heard me for months, I have never said a negative word about Elizabeth Warren, who is a friend of mine. We have differences on issues. That’s what a campaign is about.”

Some progressives believe the tension between the two candidates is being overblown.

Neither candidate has directly attacked the other. The back-and-forth has largely played out among their supporters and surrogates on social media or through leaks in the media.

“Overblown,” said Jonathan Tasini, a progressive strategist who backs Sanders. “I don’t think most voters even tuned into a spat between Twitter advocates for each candidate.”

Still, Tasini and others are warning against a full-scale war between Sanders and Warren, saying it would be detrimental to the progressive cause and potentially boost a centrist, such as Biden or Buttigieg, to victory in Iowa.

“I would only caution partisans to remember the 2004 Iowa run-up when Dick Gephardt and Howard Dean tangled over the stretch run, repelling enough voters to allow John Kerry John Forbes KerryThe Memo: Warning signs flash for Trump on debates Divided country, divided church TV ads favored Biden 2-1 in past month MORE to win the caucuses,” Tasini said. “That should be a warning sign this cycle, especially with such a large percentage of voters saying they are open to switching or haven’t made up their minds, that you do not want to be the camp seen, fairly or unfairly, as tossing a lit match into a gasoline-soaked bonfire.”

Biden allies are licking their chops.

“Is there a popcorn-eating emoji?” one ally said on Monday.

But some Democrats say the showdown between the progressive powerhouses has been a long time coming as they seek support from the same pool of liberals with the caucuses headed for an apparent photo finish.

“I think you can look at the kerfuffle between Sens. Warren and Sanders as both inevitable given the electoral calendar and as a reminder that politics and friendship are a tough combination,” said Democratic strategist Lynda Tran. “Politics is always … an exercise in comparisons. There has been endless analysis to date suggesting the policy differences between the two senators are small. Clearly [Sanders’s] team has decided drawing a contrast is critical to shoring up his numbers.”