Sanders called the story "ludicrous."
Sen. Bernie Sanders, I-Vt., told Sen. Elizabeth Warren, D-Mass., during a private meeting in December 2018 that he did not believe a woman could win the presidential election, CNN reported on Monday, citing four sources.
The New York Times and BuzzFeed also reported on the comments, which are said to have come up as the two progressive senators discussed their likely presidential bids. Neither candidate had formally announced their campaign at this point.
Sanders disputed the claim in a statement.
"It is ludicrous to believe that at the same meeting where Elizabeth Warren told me she was going to run for president, I would tell her that a woman couldn't win. 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.
He added that "of course" a woman could win. "After all, Hillary Clinton beat Donald Trump by 3 million votes in 2016," Sanders said.
Three sources told BuzzFeed that Warren herself has spoken about the remarks.
The back and forth comes one day before the two are due to share the stage at a Democratic presidential debate in Des Moines, Iowa.
The old friends had defended each other against their more moderate rivals in past Democratic debates, but cracks in their relationship began to show in recent days after Sanders started surging in the polls.
Politico reported over the weekend that the Sanders' campaign had been giving volunteers talking points describing Warren as the candidate of elites.
Warren said on Sunday she was "disappointed" by the campaign's actions.
"I was disappointed to hear that Bernie is sending his volunteers out to trash me," she told NBC News.
Sanders downplayed the Politico report, saying his campaign has "hundreds of employees" and "people sometimes say things that they shouldn't."
"I have never said a negative word about Elizabeth Warren who is a friend of mine. We have differences on issues, that's what campaigning is about. But no one is going to be attacking Elizabeth," Sanders said.
Sanders has in the past been critical of some of Warren's policy positions, particularly health care.
In anOctober interview with ABC, Sanders said his Massachusetts counterpart "is a very, very good senator. But there are differences between Elizabeth and myself. Elizabeth I think, as you know, has said that she is a capitalist through her bones. I'm not."
Before Sunday, Warren had largely avoided being critical of Sanders and his campaign, typically telling reporters asking about the differences between them that they're friends and she doesn't want to comment on other campaigns.