People familiar with the exchange said Ms. Warren walked over and told Mr. 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, Mr. 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.