Bernie Sanders has denied telling his Democratic rival Elizabeth Warren that a woman could not be elected president.
During the latest Democratic debate in Iowa, Mr Sanders rejected Ms Warren's claims that he made the remark during a private meeting in 2018 - describing it as "incomprehensible".
The pair were among six candidates sharing the stage in Des Moines as they battle it out to become the Democratic nominee who will face Donald Trump in November's presidential race.
There are just three weeks to go until Iowa becomes the first state to hold a caucus and choose their preferred candidate.
When asked about the remark during CNN's televised debate on Tuesday night, Mr Sanders said: "I didn't say it. I don't want to waste a whole lot of time on this, because this is what Donald Trump and maybe some of the media want.
"Anybody who knows me, knows that it is incomprehensible that I would think that a woman could not be president of the United States."
The 78-year-old Vermont senator pointed to a video that emerged this week, from 1988, which shows him saying that he believes a woman could be president.
Mr Sanders also said that he initially refrained from putting himself forward to be the Democratic candidate in 2016's election because of a movement backing Ms Warren to run for president.
He added: "Hilary Clinton won the popular vote by three million votes. How could anybody in a million years not believe that a woman could become president of the United States?"
The politician went on to say that if he failed to win this year's nomination he would "do everything in my power to make sure that they are elected in order to defeat the most dangerous president in the history of our country" - irrespective of whether they were a man or a woman.
Ms Warren stood by her claim that he made the comment and said she "disagreed" with it - quickly pivoting to the broader question of whether a woman could be elected president.
She said: "Bernie is my friend and I am not here to try to fight with Bernie. But look, this question about whether or not a woman can be president has been raised and it's time for us to attack it head-on."
The Massachusetts senator pointed out the men on the stage had collectively lost 10 elections, while the two women on the stage had won each election they had contested.
By the end of the intense debate, the fallout between the two long-term friends and colleagues appeared to have deepened - with Ms Warren snatching her hand away when Mr Sanders reached out for a handshake.
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In total, there are 12 Democratic candidates vying for the nomination - and they are struggling to convince voters which one of them is best positioned to defeat Mr Trump in the autumn.
Opinion polls suggest the campaign remains fluid, with Mr Sanders and Ms Warren - along with former vice president Joe Biden and former mayor Pete Buttigieg - in an extremely tight four-way race at the top.