A tense “he said, she said” dispute between Bernie Sanders and Elizabeth Warren over what the Vermont senator said at a private dinner could destroy chances of either left-wing firebrand winning the Democratic presidential nomination.

In the last weeks before the first votes in the Democratic primaries and caucuses, the two candidates appeared poised to soon push one of their main center-left competitors, former Vice President Joe Biden or former South Bend, Indiana, Mayor Pete Buttigieg, out of the race. Sanders, 78, had bounced back from an October heart attack and topped a respected Iowa poll released last week. A New Hampshire poll released Monday showed Warren, the 70-year-old Massachusetts senator, far ahead of Buttigieg.

That all changed with Warren’s description of what Sanders told her in a private December 2018 meeting.

"Among the topics that came up was what would happen if Democrats nominated a female candidate. I thought a woman could win; he disagreed," Warren said in a statement Monday, confirming earlier reports of the meeting that cited those close to the Massachusetts senator. Earlier Monday, Sanders denied the “ludicrous” allegation.

The personal swipe is a striking break from an apparent nonaggression pact between the two candidates that kept either one from attacking the other on debate stages or public statements for nearly a year. And it is a sharp escalation of a rift brewing between the two candidates over a Sanders campaign volunteer script disparaging Warren’s chances of winning, which Warren described as Sanders “sending his volunteers out to trash me.”

With each candidate adamant that their version of the conversation is correct, what was actually said can never be known. Instead, both Sanders and Warren are at risk of severely damaging their images just three weeks before the Feb. 3 Iowa caucuses, in which voters can be notoriously fickle.

While attention turns to a private personal conversation, left-wing organizations are terrified that the candidates are distracting from shared goals on policy and taking down their common enemy, centrists in the race such as Biden and Buttigieg.

“A back-and-forth about this private meeting is counterproductive for progressives,” said Adam Green and Stephanie Taylor, founders of the Progressive Campaign Change Committee that is closely aligned with Warren. “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.”

“You both are progressive champs & our movement needs to see you working together to defeat your corporate Dem opponents – not attack each other,” tweeted Democracy for America, a left-wing group headquartered in Burlington, Vermont, that endorsed Sanders's 2016 presidential bid. “Progressives will win in 2020, but only if we don't let the corporate wing or Trump divide us.”

Instead, Warren could be seen as a traitor for attacking a hero of the far-left-wing movements. Suspicions that Warren’s campaign planted the original story about the meeting with the blessing of the senator herself paint her as desperate.

Allegations of sexist comments serve to harm Sanders, who has no way to prove that the comment is wrong. It could also highlight poorly phrased statements from Sanders problematic for progressives, such as saying last year that “a lot of white folks” were “uncomfortable” voting for a black candidate or a 1972 column stating that women have rape fantasies.

Warren said that she does not want to discuss the December 2018 meeting further, but the damage of fracturing the left-wing base may be done.

“Shame on you,” Rose DeMoro, a Sanders supporter and former head of the National Nurses United union, tweeted at Warren on Monday. “[Sanders] is an honorable person and was a feminist when you were a Republican.”