Democratic voters have expressed some reservations about Mr. Sanders’s condition following his heart attack. A recent poll conducted by The Washington Post and ABC News found that only 48 percent of respondents said Mr. Sanders was in good enough health to serve as president, compared with 74 percent for Mr. Biden and 80 percent for Ms. Warren.

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“I do think his age is playing into peoples’ decision, as well as perceived electability,” said Rachel Bly, the Democratic chairwoman in Poweshiek County, Iowa. Ms. Warren, she added, was “seen as more electable.”

With that in mind, Mr. Sanders has displayed newfound determination to prove his resilience, an objective that has meant balancing his rehabilitation with concrete steps to show he has the energy, and endurance, to go the distance.

Previously known to enjoy diner food and meals at Outback Steakhouse while on the campaign trail, Mr. Sanders — with prodding from Ms. Sanders, who often travels with him — is eating more nutritiously, often asking at restaurants if they have grilled fish. He recently ate a vegan breakfast. His most common order at Panera, a lunchtime go-to on the road, is soup and a salad.

“We’re getting better eating habits and making sure we can maintain them on the road,” Ms. Sanders said in an interview last month. “For him and for me, and for everybody else on the campaign.”

He has also incorporated more physical activity into his routine, frequently asking staffers to schedule time for a walk between campaign stops. The fashion-minded have noticed the upgrade to his wardrobe: Esquire magazine recently praised his look, writing that his knitwear in particular was “smart enough for a formal dress code, but softens the angles of a tailored suit.”

And while he has long been reluctant to speak about his age, he has begun to embrace it, arguing that it gives him the upper hand.