Washington (CNN) - The uneasy peace between the two leading Democratic 2020 progressives broke down Monday night, as Sen. Elizabeth Warren publicly accused Sen. Bernie Sanders of telling her during a private meeting in December 2018 that he didn't think a woman could win.
The explosive accusation, which Sanders had previously denied in response to a CNN report, opened a chasm between the two leading liberals in the race and significantly raised the stakes ahead of Tuesday night's CNN/Des Moines Register Democratic presidential debate, the final one before voters cast their first ballots of the presidential season next month. Later on Monday night, Sanders senior adviser Jeff Weaver suggested in an interview with CNN's Chris Cuomo that Warren had misconstrued the conversation with Sanders.
The controversy raised fresh questions about the importance of identity politics in Democratic politics amid ongoing accusations that the top of the party's field isn't sufficiently diverse.
"Among the topics that came up was what would happen if Democrats nominated a female candidate," Warren said of the meeting with Sanders in a news release Monday night.
"I thought a woman could win; he disagreed."
Four sources previously told CNN that during the 2018 meeting, Sanders had told Warren he did not believe a woman could win.
The Vermont senator had expressed frustration at what he saw as a growing focus among Democrats on identity politics, according to one of the people familiar with the conversation. Warren told Sanders she disagreed with his assessment that a woman could not win, three of the four sources said.
Sanders denied the characterization of the meeting in a statement to CNN, stating 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. What I did say that night was that Donald Trump is a sexist, a racist and a liar who would weaponize whatever he could," 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."
Hours before Warren herself weighed in, Sanders campaign manager Faiz Shakir was very confident that when Warren did voice her perspective, she would agree with his view that the accusation that Sanders had told her in 2018 that a woman could not win the presidency was "salacious, ridiculous and false."
"We need to hear from her directly," Shakir told CNN earlier in the day before Warren's statement was released. "But I know what she would say: that it is not true, that it is a lie. So I mean, I welcome her coming out and disputing this and say, putting this to rest."
"You have four anonymous sources saying stuff that isn't true. There are only two people in the room. It was Elizabeth Warren, it was Bernie Sanders. They both believe that wasn't what happened," Shakir continued.
Asked how he would respond if he found out Warren did leave that 2018 meeting with the understanding that Sanders had told her that a woman could not be elected president, Shakir told CNN, "I'm not going to get into hypotheticals."
But later on Monday, in an interview on CNN's "Cuomo Prime Time," Weaver told Cuomo "there's some wires crossed here, but clearly Bernie Sanders did not say that a woman can't win."
"I think there was a discussion about Trump, misogyny, sexism in politics, and the difficulty of running in the era of Trump for women, the special challenges that women face in the era of Trump," Weaver said. "But those conversations can sometimes get misconstrued, Chris."
In her statement on Monday, Warren said she had "no interest in discussing this private meeting any further" because "Bernie and I have far more in common than our differences on punditry."
"I'm in this race to talk about what's broken in this country and how to fix it -- and that's what I'm going to continue to do. I know Bernie is in the race for the same reason," she said. "We have been friends and allies in this fight for a long time, and I have no doubt we will continue to work together to defeat Donald Trump and put our government on the side of the people."
Split between candidates at critical moment
Warren and Sanders are competing to emerge as the standard-bearer for progressives in the Democratic primary fight. The two had previously taken careful efforts to manage their friendship and political rivalry before the 2020 race was officially underway and had largely avoided publicly sniping at each other.
That's begun to change in recent days. Politico reported over the weekend that Sanders' campaign has begun attacking Warren with a memo containing a script that tells volunteers to explain that Warren is only able to attract "highly-educated, more affluent people" and unable to grow the Democratic Party's base in a way that is necessary to beat Trump.
Asked about the report, Warren replied, "I was disappointed to hear that Bernie is sending his volunteers out to trash me." Sanders has since distanced himself from the Politico report.
Democrats have faced accusations that the party is lacking diversity at the top of its presidential field. Of the six candidates who will be on the debate stage Tuesday night, all will be white, and only two -- Warren and Sen. Amy Klobuchar of Minnesota -- will be women. Democratic National Committee Chairman Tom Perez defended the party's inclusivity on CNN's "New Day" on Tuesday, saying Democrats have fielded "the most diverse field in American history" but insisting that the party can't change its standards for making debates "midstream."
Warren also has grown increasingly vocal over the past few months on the role of sexism and gender inequality in politics, including experiences the Massachusetts Democrat said she has endured in the years that she has been in public office.
The rupture of the Warren-Sanders relationship comes amid a tight race in Iowa.
Three weeks ahead of the caucuses, Warren is now the only woman polling in the double-digits nationally. A recent CNN/Des Moines Register/Mediacom poll showed Warren in a four-way race with Sanders, former South Bend, Indiana, Mayor Pete Buttigieg and former Vice President Joe Biden. In that survey, Sanders and Warren performed nearly equally well as the top choice among female likely Democratic caucusgoers, while Sanders and Buttigieg were the top choice among men.
This story has been updated with additional background on the 2020 race.