Sen. Elizabeth Warren Elizabeth WarrenWarren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon No new taxes for the ultra rich — fix bad tax policy instead MORE (D-Mass.) said in a statement on Monday that fellow presidential hopeful Sen. Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE (I-Vt.) told her in a private meeting in 2018 that a woman could not win the White House.

“Bernie and I met for more than two hours in December 2018 to discuss the 2020 election, our past work together and our shared goals: beating Donald Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE, taking back our government from the wealthy and well-connected, and building an economy that works for everyone,” Warren said in a statement. “Among the topics that came up was what would happen if Democrats nominated a female candidate. I thought a woman could win; he disagreed.”

A CNN report from earlier in the day cited four anonymous sources close to Warren claiming Sanders had told her he believes a woman cannot win the presidency.

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Sanders blasted the report, and his allies were furious, describing it as a desperate effort by Warren staffers and the mainstream media to blunt his momentum ahead of the Iowa caucuses.

"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," Sanders said.

"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. Do I believe a woman can win in 2020? Of course! After all, Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE beat Donald Trump by 3 million votes in 2016."

Tensions between the Sanders and Warren campaigns are now boiling over in the final run to Iowa after the candidates had gone a full year on the campaign trail without attacking each other.

But the latest Des Moines Register poll of Iowa finds Sanders in first and Warren close behind, ratcheting up pressure on both candidates ahead of the first-in-the-nation vote.

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Progressives are worried that the brewing battle between the party’s progressive standard-bearers will tear them both down and pave the way for a centrist candidate, such as former Vice President Joe Biden Joe BidenCast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response Biden tells CNN town hall that he has benefited from white privilege MORE or former South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBogeymen of the far left deserve a place in any Biden administration Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq A socially and environmentally just way to fight climate change MORE, to emerge victorious.

Sanders and Warren are all but certain to have to address the allegation of sexism at Tuesday night’s debate in Des Moines, although Warren said she’d like to put the matter to bed.

“I have no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences on punditry,” Warren said. “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 this race for the same reason. 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.”

Still, Sanders's supporters are fuming and passing around video clips of the former Vermont senator from the 1980s and 1990s saying that he believes a woman can be elected president.

Sanders’s allies note that the private meeting he and Warren had, during which they decided they would not attack each other, has been reported on for more than a year and that this allegation never surfaced.

And they point to reports about how Sanders encouraged Warren to run for president in 2016, when the Vermont senator challenged Hillary Clinton for the Democratic nomination.