Warren subsequently told associates about Sanders’ comment, according to people with knowledge of her remarks.

Sanders on Monday vehemently denied making the remark, which was first reported by CNN.

“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 in a statement, adding that it was “ludicrous” to think he would have made such a comment. “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 Warren campaign declined to comment Monday. 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 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, called on Warren to refute the statement.

“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 report could hinge on what happens during Tuesday’s Democratic debate in Des Moines, where Warren and Sanders will be on the same stage just weeks before the Iowa caucuses. Up to this point, the two progressive candidates have focused their attacks on more moderate rivals, a consequence of their longtime 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.

If Sanders wins the caucuses, he would have a chance to start the nominating contest with three consecutive victories, given his following in New Hampshire, where he routed Clinton in 2016, and his appeal in Nevada. Iowa is also critical for Warren, who has spent a considerable amount of time in the state and is hoping it will help sustain her online-focused fundraising and propel her deeper into the primary calendar.

Veterans of past Iowa campaigns said the report about Sanders could hurt him with some voters who are still making up their minds, particularly those deciding between him and Warren. Polls in the state indicate that Warren is the top second choice among Sanders’ supporters. Yet Democratic strategists not affiliated with either candidate urged Warren to handle the issue cautiously given that other candidates are eager to see the two progressives bloody each other.

That roster includes not just Joe Biden and Pete Buttigieg, who are bunched up with Sanders and Warren at the top of Iowa surveys, but also Sen. Amy Klobuchar of Minnesota, who has positioned herself as the best candidate to beat Trump because of her Midwestern roots.

“If I were her, I’d say, ‘Bernie and I have been friends for years and I’m not going to speak to private conversations, but of course I think a woman can win the presidency,’” said Jennifer R. Psaki, who worked on Barack Obama’s 2008 campaign.

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.

What delights Sanders’ admirers and confounds his critics, though, is that his supporters are unusually committed. His voters are more likely to say that they have made up their minds than any other candidate in the race. Episodes that would badly damage more conventional candidates — say, suffering a heart attack at 78 — do little to hurt Sanders.

“His base supporters do not waver,” said Sue Dvorsky, a former Iowa Democratic Party chairwoman who backed Sen. Kamala Harris before she dropped out.

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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.”

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 .