Elizabeth Warren has claimed Bernie Sanders told her that he did not believe a woman could defeat Donald Trump in an escalation in the growing split between the two progressive 2020 candidates.

In response to a CNN article based on four anonymous sources, Ms Warren stood by the reported account of a private meeting between the two senators but said she and Mr Sanders remained “friends and allies”.

“Among the topics that came up was what would happen if Democrats nominated a female candidate,” she said of the December 2018 meeting.

“I thought a woman could win; he disagreed.”

Mr Sanders’ team has strongly denied the claim, with campaign manager Faiz Shakir calling it a “lie”.

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

“What I did say that night was that Donald Trump is a sexist, a racist and a liar who would weaponize whatever he could,” Mr Sanders said in a statement on the allegation.

“Do I believe a woman can win in 2020? Of course! After all, Hillary Clinton beat Donald Trump by 3 million votes in 2016.”

The dispute has raised concerns among the left-wing of the Democratic Party of a public feud between Mr Sanders and Ms Warren, the only two viable progressive candidates for the 2020 election.

Although both candidates have performed well in polling so far, neither have been able to supplant former vice president Joe Biden as the frontrunner in the Democratic race.

Mr Biden currently averages about 28 per cent in polls, ahead of Mr Sanders (on about 20 per cent) and Ms Warren (on about 16 per cent), according to analysis by RealClearPolitics.

However, Mr Sanders and Ms Warren also share supporters on the Democrats' left-wing, with a Quinnipiac University poll on Monday showing 57 per cent of Sanders supporters would vote for Ms Warren and 52 per cent of Warren supporters would vote for Mr Sanders.

Meanwhile, only 20 per cent and 17 per cent of Biden supporters said they would vote for Mr Sanders and Ms Warren respectively.

In the 2018 meeting, the two candidates are said to have agreed that they would not directly attack each other during the campaign, but that agreement has frayed as the Democratic race has intensified.

Over the weekend, it was reported that the Sanders team has told volunteers to suggest Ms Warren is the candidate for “highly-educated, more affluent people” to convince voters to switch to his campaign, according to a document leaked to Politico.

Ms Warren said she was “disappointed” by the leak, which has not been denied by the Sanders campaign.

Some progressives within the Democratic Party have feared that public disagreements between the two senators could undermine attempts to elect a left-wing candidate for the Democratic nomination.

On Monday, Stephanie Taylor and Adam Green, co-founders of the Progressive Change Campaign Committee (PCCC), said that “a back-and-forth about this private meeting is counter-productive for progressives”.

“In this pivotal moment of the campaign, progressives must work together to defeat Donald Trump and prevent a less-electable establishment candidate like Joe Biden from getting the nomination,” they wrote in a statement.

Waleed Shahid, the communications director for Justice Democrats, told CNN that Mr Sanders and Ms Warren should focus their efforts on defeating candidates such as Mr Biden and avoid encouraging a fight in the progressive movement.

“Warren and Sanders should be focusing their fire on their two corporate-friendly opponents and earning the trust of voters for why their vision will defeat Trump and provide relief for the American people,” Mr Shahid said.