Elizabeth Warren accused rival Bernie Sanders of calling her a liar at the end of a Democratic candidates' TV debate, according to a newly released recording.
On Tuesday, she refused to shake his hand during a tense, post-debate talk.
The audio of their heated exchange has since emerged, revealing what the candidates said to each other.
The progressive candidates for the presidential nomination had previously refused to attack each other.
On Monday, Ms Warren alleged Mr Sanders had told her in December 2018 that a woman could not win the presidency - something Mr Sanders denies.
Asked about this during Tuesday's debate in Des Moines, Iowa, Mr Sanders denied it again - saying he had long supported the idea of a female president.
They appeared to end the night on a tense, frosty exchange, which could not be heard on the live broadcast at the time.
But sound of the moment was caught by CNN's back-up audio system and found later on Wednesday.
"I think you called me a liar on national TV," Ms Warren can be heard saying.
"What?" Mr Sanders responds.
"I think you called me a liar on national TV," she repeats.
"You know, let's not do it right now. If you want to have that discussion, we'll have that discussion," he says, to which she replies: "Anytime."
"You called me a liar," he continues. "You told me - all right, let's not do it now."
Aides to the two candidates have for days attempted to de-escalate the feud amid fears that it could ultimately hurt them both and benefit more centrist Democratic hopefuls such as former Vice-President Joe Biden, the Associated Press reports.
This was the final televised debate for Democrats hoping to be nominated as their party's candidate to run for the US presidency.
The Iowa caucuses on 3 February kick off the presidential primary season, as the candidates battle for the right to challenge President Donald Trump in November's election.