Elizabeth Warren took the unusual step of confirming a report that fellow Democratic White House contender Bernie Sanders told her during a 2018 meeting that he did not believe a woman could win the 2020 presidential race.
Ms Warren and Mr Sanders are fellow US senators, friends and their party's progressive standard-bearers who agreed early in the nominating contest to an informal non-aggression pact.
But amid escalating tensions between the two rivals and with the first nominating contest looming in early February, Ms Warren pushed back against Mr Sanders' denial of a CNN report detailing the meeting in which he told her he did not believe a woman could beat Republican President Donald Trump.
"I thought a woman could win; he disagreed," Ms Warren, a US senator from Massachusetts, said in a statement released late on Monday describing the two-hour meeting in December 2018.
Ms Warren and Mr Sanders are among the six candidates who will face off on Tuesday in the seventh Democratic presidential debate, and the last before the Feb. 3 Iowa caucuses, which kick off the state nominating contests.
In its report on the meeting, CNN cited four people with knowledge of it, including two with whom Ms Warren spoke about it soon after and another two who were familiar with what happened at the meeting.
Mr Sanders, a US senator from Vermont, told CNN it was "ludicrous" that he would tell Ms Warren a woman could not win. "What I did say that night was that Donald Trump is a sexist, a racist and a liar who would weaponise whatever he could," he added.
Mr Sanders' campaign did not immediately respond to a request to comment on Ms Warren's confirmation of the CNN account.
"I have no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences in punditry," Ms Warren said.
The tussle between the two progressive senators is the second in as many days after Politico reported on Sunday that Mr Sanders' campaign had distributed talking points for volunteers on what to say to voters who are thinking of supporting his main rivals.
The guidance suggested that volunteers argue that Ms Warren was supported by "highly-educated, more affluent people who are going to show up and vote Democratic no matter what."
In response, Ms Warren, while campaigning in Iowa, said she was "disappointed" in Mr Sanders' campaign and hoped he reconsidered the approach.
Mr Sanders said he had not approved the negative talking points related to his Democratic rivals.