CEDAR RAPIDS, Iowa ― Tensions between the presidential campaigns of Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders, the two leading progressive candidates for the Democratic presidential nomination, are at an all-time high after the two candidates’ accounts of a 2018 meeting sharply diverged.

In a Monday night statement, Warren confirmed a CNN report that Sanders had told her in a late 2018 meeting that a woman could not win the presidency. “I thought a woman could win; he disagreed,” she said.

The Sanders campaign has steadfastly denied the Vermonter said anything of the sort.

“It is a lie. Bernie Sanders has always stood for women and women’s rights,” Sanders campaign manager Faiz Shakir told CNN.

The fight between the two progressive New England senators was the second in as many days and seemed to effectively end a truce that had held since the earliest days of the presidential race. It followed a dustup over a Politico story showing a Sanders campaign script for volunteers casting Warren as the candidate of the “affluent” who would bring “no new bases into the Democratic Party.”

Both campaigns have largely declined to officially elaborate on the escalating disputes, which come three weeks ahead of the all-important Iowa caucuses and just one night ahead of a debate in Des Moines. A Des Moines Register/CNN poll of the caucuses, released Friday, showed Sanders leading the contest, with Warren, former Vice President Joe Biden and former South Bend, Indiana, Mayor Pete Buttigieg closely behind.

While the battle is relatively tame by the standard of past presidential primaries — neither Warren nor Sanders has attacked the other with paid media or incorporated critiques into their stump speeches — the possibility of a broader clash between them has alarmed many progressive organizations, who fear the fight’s only beneficiary would be the more moderate Biden.

Joe Biden is like the final boss in a video game, and the progressive candidates are rapidly eroding their health bar by fighting each other. Sean McElwee, co-founder of Data For Progress

Biden’s lead in national polling ― and especially his strong standing with Black voters ― has made him the front-runner for the Democratic nomination.

“I think there’s a view among some people in the progressive community that Joe Biden will be very easy to beat,” said Sean McElwee, the co-founder of Data For Progress and an occasional adviser to Warren. “But Joe Biden is like the final boss in a video game, and the progressive candidates are rapidly eroding their health bar by fighting each other.”

Democracy for America, another progressive group, has been scrambling to get the two campaigns to lower the temperature, with little apparent success so far.

“There are only two candidates who are ready to fight for Medicare for All and the Green New Deal, and those are Elizabeth Warren and Bernie Sanders. And we need them working together,” said Charles Chamberlain, the group’s chair. “Don’t let the corporate wing or Donald Trump divide us.”

Chamberlain suggested the two campaigns should work together to quickly end the dispute: “Just like this thing can blow up out of nowhere, they can put it out very quickly,” he said. “We don’t see Biden and Buttigieg fighting each other right now. They understand that’s not in their best interest.”

Mike Blake / Reuters Former Vice President Joe Biden would likely welcome a fight between Vermont Sen. Bernie Sanders and Massachusetts Sen. Elizabeth Warren at Tuesday night’s debate in Des Moines, Iowa.

But for a number of reasons, the dispute is unlikely to end anytime soon. While Warren’s statement seeming sought to downplay the rift, saying she had “no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences on punditry,” Tuesday night’s moderators from CNN and the Des Moines Register are likely to probe. And Sanders’ allies appear divided on whether or not to accept the seeming olive branch.

The Progressive Change Campaign Committee, a group that has long functioned as Warren’s unofficial political arm, suggested in a statement on Monday night that “that a back-and-forth about this private meeting is counter-productive for progressives.”

“In this pivotal moment of the campaign, progressives must work together to defeat Donald Trump and prevent a less-electable establishment candidate like Joe Biden from getting the nomination,” said Adam Green and Stephanie Taylor, the group’s co-founders.

Larry Cohen, the chair of the Sanders-backed Our Revolution, has suggested that both Sanders and Warren should stay in the race as long as possible to maximize the number of delegates committed to a progressive candidate at the Democratic national convention. He was hopeful both candidates could bury the hatchet

“Elizabeth Warren’s statement points to that, not some continuing rift,” he said Monday night, suggesting that the two campaigns should engage in some public display of unity: “I think something like that would be helpful.”

Other Sanders allies were less forgiving.

“Shame on you,” Rose DeMoro, the former head of National Nurses United and a prominent Sanders backer, tweeted at Warren, saying Sanders “was a feminist when you were a Republican.” (Warren was a registered Republican until the mid-1990s.)

And even beyond the likelihood of a debate showdown, the staying power of each campaign means the two senators can no longer hope the other will gently fade into irrelevance. During Warren’s long and steady gains over the summer and early fall, Sanders’ numbers barely nudged, providing her campaign with the hope she could consolidate the left. Warren’s subsequent dip in polling provided Sanders’ team with the hope she would be obscure by the time the first votes were cast, but her resiliency has left both campaigns with little choice but to battle for left-leaning dominance.

Just like this thing can blow up out of nowhere, they can put it out very quickly. Charles Chamberlain, Democracy for America

Polling indicates both could pick up a majority of the other’s voters if they can manage to land a knock-out punch. In a Quinnipiac University poll released Monday, Biden led the field with 25% of the vote, followed by Sanders at 19% and Warren at 16%, with every other candidate receiving less than one-tenth of the vote. But when asked who their second choice was, 52% of Warren supporters said Sanders and 57% of Sanders supporters said Warren. Biden, comparatively, would pick up 12% of Warren supporters and 11% of Sanders supporters.

And both campaigns have accumulated a year’s worth of grievances from the truce period. As far back as June, the Sanders campaign would occasionally cast Warren as a member of the “corporate wing of the Democratic primary,” with aides going even farther. And in recent days, the Warren campaign has aggressively played up her ability to unite the Democratic Party ― implicitly arguing that baggage from Sanders’ 2016 campaign against Clinton will prevent him from doing the same. Warren herself invoked the “factionalism” that resulted from the 2016 race when responding to Sanders’ volunteer script.

Warren allies have begun citing her traditional refusal to attack other Democrats as a reason she could unite the party while Sanders ― who has attacked Biden near-constantly since the race’s beginning ― could not.

“Caucusgoers, they see that she’s run a positive campaign, they see that she hasn’t been attacking other candidates,” said California Rep. Katie Porter (D), a Warren backer and Iowa native who was attending a house party for the campaign in Cedar Rapids. “That is important to unifying the party. We’re not going to win by dividing against each other ― we have to stand together. And stand together and then grow those who stand with us.”

Attendees at the party ― which took place before the CNN report on Sanders’ comments ― largely said they hoped Warren and Sanders would keep the peace.

“It’s a really touchy one because Bernie supporters can be Warren supporters in a heartbeat, and I don’t think you win them over by going after him,” said former state Sen. Steve Sovern (D), who hosted the house party with Porter and had backed Sanders in 2016. “And there’s not much to go after him on except for age or crankiness and stuff that are personality traits. I think she’s just got to wait it out.”

Warren’s campaign may have made a different decision.