Alexandria Ocasio-Cortez Endorses Bernie Sanders at New York Rally

Beyond serving as a show of strength, their message aligned with one his campaign is aggressively trying to project: That he is building a multi-racial, working-class coalition of voters; that his campaign speaks to men and women, to white voters and minorities.

It is a message he needs now more than ever.

The endorsement jolted the primary race, signaling that Mr. Sanders, 78, was still a formidable contender just as it had increasingly seemed like a contest between Senator Elizabeth Warren and former Vice President Joseph R. Biden Jr. They also shifted the conversation away from his health issues and his age, infusing his campaign with a renewed sense of vitality.

“There’s been some degree of criticism overall — Bernie Sanders can’t win because his movement is tapped out,” Faiz Shakir, Mr. Sanders’s campaign manager, said. “This discounted that.”

But if the endorsements were an obvious indication that Mr. Sanders was not ready to surrender the party’s left flank to Ms. Warren, it is not clear how much they will ultimately change the race — in part because there are signs that voters are not taking their cues from endorsements. Ms. Warren, for instance, has attracted huge crowds, posted some of the biggest fund-raising numbers, and surged to the top of national and early-state polls despite lacking endorsements from a single governor, big-city mayor or senator outside her home state. At the same time, Senator Kamala Harris of California is struggling to gain momentum even though she has the backing of politicians across the country, including her state’s governor, Gavin Newsom.

Mr. Sanders’s endorsements could inject fresh energy into a campaign that in some respects needed it badly. Consistently trailing Mr. Biden and Ms. Warren in recent polls and struggling to expand his base, he spent the last two weeks facing a barrage of questions about his health. His campaign made a show of financial strength this month when it reported it had collected $25.3 million between July and September — the most of any candidate in that period — but the announcement was quickly eclipsed by the news of his heart attack.

Mr. Sanders’s campaign is hoping that endorsements from Ms. Ocasio-Cortez and Ms. Omar — and possibly Representative Rashida Tlaib of Michigan— prove that he is building a multiracial, working-class coalition of voters. His aides are also confident that the women will motivate young people, a group that was critical to his success in 2016 and that his allies know he must win over again, both optically and for actual votes.