In a private meeting in 2018 between Senator Elizabeth Warren of Massachusetts and Senator Bernie Sanders of Vermont, Mr. Sanders said that he did not think a woman could win the presidency, according to two people familiar with the discussion.
Ms. Warren subsequently told associates about Mr. Sanders’s comment, according to people with knowledge of her remarks.
Mr. 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,” Mr. 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 three million votes in 2016.”
Mr. Sanders added that he had told Ms. Warren “that Donald Trump is a sexist, a racist and a liar who would weaponize whatever he could.”
The Warren campaign declined to comment on Monday.
Ms. Warren has spent much of the campaign praising Mr. Sanders or aligning herself with him on some policies, including universal health insurance, but tensions between their two campaigns have increased in recent days as the Iowa caucuses neared.
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 Mr. Sanders offered the comment while giving his assessment on the coming race. In relaying to Ms. Warren the challenges he thought her campaign would face, he not only said that President 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 Mr. Sanders, said that Mr. 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,” Mr. Cohen said.
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 Ms. 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 Mr. Sanders had urged her not to run, Ms. Warren said, “Bernie and I had a private dinner, and my view is that dinner stays private.”
For months, Ms. Warren and Mr. Sanders, the most prominent progressive candidates in the presidential race, abided by a tacit nonaggression pact. But that pact has begun to fray.
Over the weekend, Politico reported on a script distributed to volunteers for the Sanders campaign that suggested telling backers of Ms. Warren to support Mr. 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, Ms. Warren said that Mr. 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 Mr. Sanders faced off against Mrs. 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.”
Mr. Sanders said that he had never personally attacked Ms. 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 on 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.”
Sydney Ember and Maggie Astor contributed reporting.