The inevitable question Sanders received from moderators about his age was relatively brief and painless. Asked how he would reassure Democratic voters that he has the energy to be president after his heart attack, he urged them to come to his rally in New York City this weekend, which drew laughter from some in the crowd.

“We are going to be mounting a vigorous campaign all over this country,” he said. “That is how I think I can reassure the American people.”

It left the impression that, at least for tonight, Bernie is still Bernie.

Sanders lucked out that the moderators didn't ask him why his campaign failed to disclose his heart attack for nearly three days, a delayed response that's drawn criticism from some quarters.

Toward the end of the debate, Sanders took a shot at Joe Biden, whom some of his aides and fans have wanted him to attack more forcefully. When Biden said that he was the only candidate who “has gotten anything really big done,” Sanders delivered this rejoinder: "But you know what you also got done — and I say this as a good friend — you got the disastrous war in Iraq done. You got a bankruptcy bill, which is hurting middle class families all over this country. You got trade agreements like NAFTA and [permanent normal trade relations] with China done, which have cost us 4 million jobs."

Sanders had sunk to third place in national polling even before his heart attack put an unwanted spotlight on his age. At 78, he is the oldest candidate in the race.

Following his health scare, a HuffPost/YouGov poll found that 59 percent of registered voters believe that Sanders' physical condition is not good enough for him to serve effectively as president. That's a figure Sanders has to turn around in the coming weeks.

In recent days, Sanders and his aides have said that he feels great and would show that he has the energy to be commander-in-chief through a strong debate performance. A Sanders staffer said before the debate that “standing against the odds is something our working-class voters love to see.”

Doctors inserted two stents to address a blockage in one of Sanders’ arteries — a procedure that physicians say often leaves patients feeling more vigorous.

Some of Sanders’ advisers have also pushed him to use his heart attack as a way to share a personal side of himself with voters, something he doesn’t often do on the trail. They’ve pointed to Franklin Delano Roosevelt as an example of a president who pushed past setbacks to help ordinary Americans.

In TV interviews and on social media ahead of the debate, Sanders said he thought about what would have happened if he didn’t have good health insurance while “lying in a hospital bed.” However, he largely avoided talking about his personal life on Tuesday, aside from saying that he was touched by the response he received from the public after his heart attack.

“Let me take this moment, if I might, to thank so many people from all over this country, including many of my colleagues up here, for their love, for their prayers, for their well wishes,” he said.

Sanders’ staff said he prepared for the debate by huddling with aides for days in Burlington, Vermont. His team also said that he helped build up his endurance by playing baseball and spending time with his grandchildren. Sanders does not typically participate in mock debate sessions.

Sanders may benefit tonight from relatively low expectations from the media and voters. Only 12 percent of Democratic primary voters said they expected him to perform the best of any candidate, compared to 31 percent tho thought that of Elizabeth Warren and 25 percent of Joe Biden, according to a poll by POLITICO/Morning Consult.