Both Ms Warren and Mr Sanders are vying for their party’s nomination in the forthcoming Presidential elections in 2020. The pair represent the so-called progressive wing of their party, and are on the same page when it comes to the policies they want to see implemented. They have argued that the country needs sweeping changes to health care, financial and other systems and plan to tax higher-income and wealthier Americans to create new benefits for everyone else.
The Democratic Senators have a long-standing pact to refrain from attacking each other.
However, the pact appears to be lying in tatters, after Ms Warren accused Mr Sanders of calling her a liar on national TV.
The heated confrontation was picked up by CNN microphones after the debate had concluded.
At the conclusion of the televised event, the Massachusetts Senator made her way over to Mr Sanders.
She pointedly refused to shake his hands, before challenging him over remarks he made during the debate.
Ms Warren can be heard saying: “I think you called me a liar on national TV.”
When the stunned Vermont senator challenged her, Ms Warren repeated her initial allegation.
A clearly irritated Mr Sanders then replied: “You know, let’s not do it right now.
JUST IN: South China Sea: How Iran feud and Australia fires could help Beijing
He insisted the comment was pure fiction and had been made up by “staff who weren’t in the room and are lying about what happened.”
Mr Sanders added: “It is ludicrous to believe that at the same meeting where Elizabeth Warren told me she was going to run for president, I would tell her that a woman couldn’t win.
“Do I believe a woman can win in 2020? Of course!
“After all, Hillary Clinton beat Donald Trump by 3 million votes in 2016.”
Ms Warren issued her own statement later on Monday, where she stood by her allegation.
In the statement, she said: “Among the topics that came up was what would happen if Democrats nominated a female candidate.
“I thought a woman could win, he disagreed.”
The Iowa event is the latest in a series of debates, as the remaining six candidates fight it out to secure their party’s nomination for the November election.
The contest is expected to go to the very end, with the winner to be declared at the Democratic Convention in Milwaukee, Wisconsin, in July.