Elizabeth Warren appeared to snub a handshake from Bernie Sanders at the end of a Democratic presidential debate in which previously genial ties between the pair began to fray.

Footage showed Ms Warren turn to engage the Vermont senator in a seemingly terse exchange, either ignoring or failing to notice his outstretched hand in the process.

“I felt like ‘okay, there’s something going on here – good night, I’m out of here’,” said bystander and fellow candidate Tom Steyer, who added: “You could see it was an awkward moment.”

With less than three weeks before the Iowa caucus kickstarts the official nomination process, Ms Warren and Mr Sanders’ longstanding ceasefire in their rival progressive bids may be starting to crack under the pressure.

The fissure first appeared after Politico reported that the Sanders team were instructed to warn voters against supporting Warren. Then, ​sources told CNN that when Ms Warren informed Mr Sanders of her intention to run for president at a private meeting in 2018, he had responded that he didn’t believe a woman could win.

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

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

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

Ms Warren’s communications manager Kristen Orthman initially declined to comment on the allegations, but on Tuesday Ms Warren released a statement, saying of the conversation: “I thought a woman could win; he disagreed.”

Ms Warren maintained the two were longtime “friends and allies” and would continue to work together, saying she would not discuss further discuss the matter, which she described as “our differences in punditry”.

While both parties appear to have surmised the fracas would do little to benefit either of their causes, the controversy came to the fore during Tuesday’s debates, when the moderator asked Mr Sanders: “You’re saying that you never told Senator Warrant that a woman could not win the election?”

“That is correct,” he replied, before the moderator immediately asked: “Senator Warren, what did you think when Senator Sanders told you a woman could not win the election?”

While the framing of the question drew laughter from the Vermont senator and the audience, later evolving to outrage from his supporters on social media, Ms Warren responded: “I disagreed.”

Neither candidate explicitly referred to the accusation again during the debate, but Ms Warren used the issue to immediately pivot into a series of apparent set pieces on the subject.

Firstly, she argued that herself and the only other female candidate, Amy Klobuchar, had enjoyed more electoral success than their male counterparts saying: “Look at the men on this stage. Collectively, they have lost 10 elections.”

Elizabeth Warren: 'The only people on this stage who have won every single election that they've been in are the women'

Continuing to reference the allegations in a roundabout way, she framed a female victory in historic terms.

“Look don’t deny that the question is there. Back in the 1960s people asked could a Catholic win,” she said, referring to John F Kennedy. “Back in 2008 people asked if an African American can win. In both times the Democratic Party stepped up and said yes, and got behind their candidate.”

And, finally, she took a backhanded aim at the DNC’s apparent fear that Mr Sanders is too divisive.

“The real danger we face as Democrats is picking a candidate who can’t pull the party together or someone who takes for granted big parts of constituency,” she said. “We need to excite all parts of party, bring everyone in and give everyone a Democrat to believe in.”