Senator Bernie Sanders and his advisers were not anxious before the first three televised primary debates, confident in his ability to champion his ideas and dubious that the debates would dramatically change how voters regarded him.
But his aides know the next debate will be different: When he takes the stage on Tuesday night, two weeks after suffering a heart attack, he will have to convince voters that he is strong enough to carry the party’s challenge to President Trump next fall.
Since he experienced chest pains on Oct. 1 at an event in Las Vegas and had two stents inserted to clear a blocked artery, Mr. Sanders, 78, has remained off the campaign trail, first recovering at a hospital and then at his home in Burlington, Vt.
News that his campaign had waited several days to reveal his diagnosis of a heart attack — and the limited medical details that they have released since — has also raised questions of transparency and contributed to speculation about his condition.
His campaign has not made his doctors available for interviews, and while Mr. Sanders has pledged to release his medical records, he has not yet done so.
His health issues have cast a degree of uncertainty over his campaign and left his aides rushing to reassure voters about his age and health, just as he was trying to improve his standing in a race that in recent weeks has become more of a two-person contest between Senator Elizabeth Warren and former Vice President Joseph R. Biden Jr.
But if debates are an endurance test for all candidates, Tuesday’s event has now taken on new urgency for Mr. Sanders, as he seeks to dispel doubts and show a national audience that he is physically capable of competing for the Democratic nomination.
“He’s got to step back onto the stage and show that he’s vigorous and healthful,” said Mark Longabaugh, a Democratic strategist who worked for Mr. Sanders’s campaign in 2016 but left his team earlier this year. “I think he has only one objective in this debate, and that’s just to show that he’s healthy.”
Mr. Sanders has been preparing for the debate at his home in Burlington, where he has largely followed his typical pre-debate playbook: Rather than conducting intensive drills and mock debates — which he and his team generally do not view as productive given how many candidates will be onstage — Mr. Sanders has been having conversations with his close advisers, including his wife, Jane Sanders, about policy and his record.
At Mr. Sanders’s insistence, however, much of his debate preparations this time have occurred outdoors, in his backyard, according to a campaign official. He has alternated between sitting and standing, and during breaks, he has gone for walks or batted around a baseball to build up his endurance.
Aides have said they expect Mr. Sanders to come across as energetic as ever and have rebuffed questions about whether he can handle standing for so long onstage. But they also allow that Mr. Sanders, his health and his appearance will be under intense scrutiny. (During the last debate, he struggled with a hoarse voice.)
His team expects the topic of his heart attack to come up at the debate and anticipates he will be asked about it by the moderators.
Mike Casca, a spokesman for the Sanders campaign, said in a statement that Mr. Sanders “will do what he’s always done” at the debate.
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“I promise the pressure of doing right by the working class on Tuesday night is bigger than any boring conventional wisdom about the importance of his ‘performance’ pushed by political pundits in D.C.,” Mr. Casca said. He added that “the solidarity forged over the last two weeks is going to make his coalition stronger.”
Several staff members, almost verbatim, have said the episode, though initially concerning, has empowered them with a new sense of resolve, heightening the sense of unity.
“More grateful than ever to be part of the @BernieSanders movement and family,” Josh Orton, a senior adviser, wrote on Twitter on Saturday.
Mr. Sanders’s campaign recently said it had raised $25.3 million in the third quarter, more than any other candidate, which has also helped buoy morale.
Officials describe the strategy going forward as business as usual. The campaign has continued to release policy plans, including one last week on campaign finance.
While Mr. Sanders recovers, an army of campaign surrogates has fanned out across the country to make his case for him. Last week, he released a seven-minute, direct-to-supporter video reaffirming his commitment to his political revolution and overcoming adversity. He has promised to continue his fight, especially for his signature policy proposal, “Medicare for all.” Campaign leaders have been providing campaign updates and rallying staffers on conference calls.
The campaign is also attempting to portray his return to the campaign trail as a comeback story, and will hold a “Bernie’s Back” rally next weekend in New York City. To the extent he has talked about his heart trouble, he has framed it as a common medical procedure that many people have experienced.
“People are back on their feet pretty soon, as is the case with me,” he said in an interview that aired on ABC News on Sunday.
In that interview he also drew an explicit contrast to Ms. Warren, a tactic he has mostly avoided.
“Elizabeth, I think, as you know, has said that she is a capitalist through her bones. I’m not,” he said in response to a question about what he would say to voters who might prefer Ms. Warren, his chief ideological rival, because she is younger and did not just have a heart attack.
His campaign, though eager to highlight how Mr. Sanders differs philosophically from Ms. Warren, said the comment did not reflect a broader change in strategy.
Yet despite his campaign’s projection of optimism, the episode has also exposed the uncertain nature over this next phase of his candidacy. Speaking to reporters last week, Mr. Sanders, who was known for keeping a grueling schedule on the campaign trail, said he intended to “change the nature” of his campaign, holding fewer events than he had previously.
But aides immediately downplayed his remarks and by the next day, Mr. Sanders told NBC News that he had “misspoke” when he implied he would be slowing down the pace of his campaign and vowed to “get back into the groove of a very vigorous campaign.”
Several staffers, however, said they were relieved he would be reducing the number of events, even if the slowdown was only temporary.