But in January, their relationship appeared to cool after she confirmed a report that Mr. Sanders had told her in private that he did not believe a woman could win the presidency in 2020. He denied that at a debate, to which she replied afterward, “I think you called me a liar on national TV.” She and her allies faced a torrent of backlash online from supporters of Mr. Sanders, and while he has condemned such vitriol, some Warren voters are wary of boosting the Vermont senator’s campaign.

While Ms. Warren, 70, is more ideologically aligned with Mr. Sanders than she is with Mr. Biden, that doesn’t apply to all of her supporters. Many are college-educated women who were drawn to Ms. Warren for her energetic, intellectual style and long list of credentials. While her supporters generally embraced her leftward message, some may be uncomfortable with Mr. Sanders’s calls for political revolution.

“I do not think it’s a foregone conclusion they all go to Bernie,” said Jess Morales Rocketto, a progressive strategist who worked on Hillary Clinton’s campaign. “Or that even if Warren endorses Bernie, that all of her voters consolidate. Some of them might go with Joe Biden. More of them than people understand are up for grabs. This is a close race. Candidates should work hard to get their vote.”

Does she endorse Biden?

Ms. Warren and Mr. Biden have had strong, longstanding disagreements over issues ranging from bankruptcy law to health care. In the fall, they traded sharp words more directly: He cast her as a “my way or the highway” elitist, and she suggested he was running in the wrong party. And they hold fundamentally different views about how to govern in Washington: She has called for “big, structural change,” and he has advocated a more incremental approach rooted in bipartisan compromise.

An endorsement of Mr. Biden would be deeply disappointing to some of her supporters who relish her persona as a “fighter.”