Fellow progressives Elizabeth Warren and Bernie Sanders have sparred on the debate stage in Iowa over the issue of electability, with the Massachusetts senator invoking the likes of Barack Obama and John F Kennedy to argue that America can elect a woman president.

While Mr Sanders denied the accusation that he told Ms Warren in 2018 that he does not believe a woman can be president, Ms Warren stood her ground and said during the final debate in Iowa before the state's caucuses next month that she had disagreed with the Vermont senator when he allegedly said as much to her.

Then she attacked the suggestion that a woman couldn't be elected: “Look at the men on this stage. Collectively, they have lost 10 elections. The only people on this stage who have won every single election that they've been in are the women. Amy and me," Ms Warren said, referring to the other woman on the stage, senator Amy Klobuchar.

Throughout the exchange, Mr Sanders stood his ground and suggested it was unimaginable that he would say he does not believe a woman can be president. He referred to his decision to hold back on entering the race in 2016 amid an effort to draft Ms Warren to run herself, and noted that there are clips of him discussing the importance of a female presidency decades ago.

"Who believes a woman can’t win? Of course a woman can win. The real question is how do we beat Trump?" Mr Sanders said before arguing that his candidacy is best poised to do so in the general because of its persistent energy.

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Ms Warren, who earlier declined to attack Mr Sanders personally, agreed with Mr Sanders on that question, before arguing that the 2018 election gains for Democrats were fuelled by women candidates and voters.

"Look don’t deny that the question is there. Back in the 1960s people asked could a Catholic win," she said, referring to former president 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."

The issue marked a rare moment of tension between the two progressives, who lead in the recent CNN/Des Moines Register poll of Iowa voters.

The pair had previously been chosen to team up on the debate stage, and respond to attacks form more centrist candidates while leaving the other alone.

But, in the days leading up to the Tuesday debate, Mr Sanders' campaign appeared to try to undercut her campaign by giving out a script to volunteers that accused Ms Warren of being a candidate that primarily appeals to wealthy elites.