People familiar with the exchange said Warren walked over and told Sanders that she was concerned that, during the debate, he had mischaracterized a conversation they had in 2018 about whether a woman could win the presidency. She has accused him of saying that a woman could not; he has denied that remark.

Appearing frustrated, Sanders asked to discuss the matter at a different time, said the people, who insisted on anonymity to discuss a sensitive, private conversation. He pointed his finger toward her, then back at himself, before turning and walking away.

Both the Warren and Sanders campaigns declined to comment on Wednesday.

The exchange, video of which was captured by CNN’s cameras, came as tension has escalated between the two leading progressives, after they labored for the past year to abide by a nonaggression pact.

Over the weekend, Warren said she was “disappointed” in Sanders after Politico reported that his campaign had distributed a script to volunteers suggesting she appealed mainly to highly educated voters. On Monday, CNN reported that Sanders had told Warren in a private meeting in 2018 that he thought a woman could not win the presidency; Sanders vehemently denied it.

“I thought a woman could win; he disagreed,” Warren said in a statement Monday.

On Tuesday, the issue burst forth onto the debate stage in a remarkable moment before a national audience that captured the recent friction between the two senators.

“I didn’t say it,” Sanders insisted, about her characterization of his 2018 remarks. Warren disputed that, then called him her friend before pivoting to make the case that of the six candidates onstage, only the women had won all of their elections.

CNN did not capture sound of the brief exchange, though it appeared that Tom Steyer, the billionaire businessman, stood close enough to overhear what was said. After the debate Steyer repeatedly denied having heard the back-and-forth.

“I was just saying good night to the two of them,” he said. “I didn’t hear anything.”

This article originally appeared in The New York Times .