Sanders vehemently denied making the remark earlier Monday and accused the Warren campaign staff of “lying” about it, in a statement intended to refute a news report by CNN that relied on anonymous sources. The New York Times and other outlets confirmed the CNN report Monday afternoon, while the Warren campaign initially declined to comment.

With the two Democrats set to face off Tuesday night in their next televised debate, they now find themselves in the most tense standoff of the Democratic nomination fight: Sanders insisting he did not make the explosive remark, which risks alienating many Democrats, and Warren saying on the record that he did make the comment.

Warren, in a statement Monday night, said that Sanders made the remark at a two-hour meeting in December 2018 where they discussed the 2020 election and “our past work together and our shared goals,” in particular defeating President Donald Trump.

“Among the topics that came up was what would happen if Democrats nominated a female candidate. I thought a woman could win; he disagreed,” she said. “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 added that she and Sanders were “friends and allies” and said she believed they would continue to work together to beat Trump.

Internally, the Sanders campaign was roiled by Warren’s statement, which seemed to catch several staff members off guard.

Her statement represents a startling break in the Warren-Sanders relationship, which has largely been cordial and mutually beneficial. The two are both leading candidates in Iowa, where they are fighting for a similar voter slice, and the back-and-forth will most likely animate not only Tuesday night’s debate but the jockeying among the liberal groups and activists who are aligned behind either candidate.

Warren’s advisers insisted that she had no intention of making this private meeting a public spectacle. However, after Sanders’ team forcefully denied the reports, some advisers said they believed she had no choice but to offer her account firsthand.

Sanders, in his statement earlier Monday, said it was “ludicrous” to think he would have made such a comment.

“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,” Sanders said. “Do I believe a woman can win in 2020? Of course! After all, Hillary Clinton beat Donald Trump by 3 million votes in 2016.”

Sanders added that he had told Warren “that Donald Trump is a sexist, a racist and a liar who would weaponize whatever he could.”

The people familiar with the 2018 meeting, who were briefed on it shortly after it took place but were not authorized to speak publicly, said Sanders offered the comment while giving his assessment on the coming race. In relaying to Warren the challenges he thought her campaign would face, he said not only that Trump would weaponize sexism, but also that such attacks would preclude a woman from being elected, according to the private accounts.

Larry Cohen, a longtime friend and adviser to Sanders, said earlier Monday that Sanders had told him about the meeting after it happened and that he did not believe the report.

“Everything I know about Bernie Sanders for 30 years tells me he would never speak like that, let alone to a woman he admires tremendously,” Cohen said.

Faiz Shakir, Sanders’ campaign manager, had called on Warren on Monday afternoon to refute the accusation.

“We need to hear from her directly,” Shakir said, “but I know what she would say — that it is not true, that it is a lie.”

The existence of the meeting has been public since shortly after it happened in December 2018. The New York Times reported shortly after the meeting took place that Warren had sought it “as a courtesy,” and that neither party had tried to gain the other’s support or discourage the other from running. But the two senators were the only people in the room, and all reports of what was said have been secondhand.

Asked last March whether Sanders had urged her not to run, Warren said, “Bernie and I had a private dinner, and my view is that dinner stays private.”

The full political effect of the controversy could hinge on what happens during Tuesday’s debate in Des Moines, Iowa. Up to this point, Warren and Sanders have focused their attacks on more moderate rivals, a consequence of their friendship and a desire to not damage the party’s left wing.

However, tensions between the two campaigns — and among their most fervent supporters — have increased in recent days. One well-respected poll from The Des Moines Register and CNN placed Sanders and Warren as the top two candidates in Iowa, with Sanders leading the pack.

Beyond how the candidates grapple with the issue is the matter of how exactly it is perceived — as a sexist gibe by a candidate whose 2016 campaign faced complaints from female staff members, an 11th-hour smear by a candidate who has lost ground or some combination of both.

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Over the weekend, Politico reported on a script distributed to volunteers for the Sanders campaign that suggested telling backers of Warren to support Sanders instead. “The people who support her are highly educated, more affluent people who are going to show up and vote Democratic no matter what,” the script said. “She’s bringing no new bases into the Democratic Party.”

Responding to the leaked script, Warren said that Sanders had been “sending his volunteers out to trash me.”

She urged against repeating the “factionalism” seen among Democrats during the 2016 primary, in which Sanders faced off against Clinton. “We can’t have a repeat of that,” she said. “Democrats need to unite our party and that means pulling in all parts of the Democratic coalition.”

Sanders said that he had never personally attacked Warren, and that both campaigns had hundreds of employees, who “sometimes say things that they shouldn’t.”

Late Monday, Rep. Tulsi Gabbard of Hawaii, another Democratic presidential candidate, wrote on Twitter that she had also met with Sanders before announcing her bid.

“We had a nice one-on-one conversation and I informed him that I would be running for president,” she said. “In that meeting, he showed me the greatest respect and encouragement, just as he always has.”

Democracy for America, a prominent progressive group, expressed alarm Monday at the growing tensions.

“You both are progressive champs & our movement needs to see you working together to defeat your corporate Dem opponents — not attack each other,” the group tweeted. “Progressives will win in 2020, but only if we don’t let the corporate wing or Trump divide us.”

This article originally appeared in The New York Times .