But, he added, "I certainly intend to be actively campaigning" and "I don't know that there's anybody that did more rallies than we have done."

Sanders also said Tuesday that the heart attack that has briefly sidelined him from the campaign trail did not mean he would be moving up the timeline to release his medical records, telling reporters they would come out “at the appropriate time.”

“We always planned to release them and we have more medical records, obviously, now” after the heart attack.

Last week's heart attack, which Sanders' campaign disclosed nearly three days after announcing he’d been hospitalized and received two stents to address a blockage in an artery, has thrust concerns about the age of Democrats’ top presidential contenders out into the open.

The senator has vowed he’ll be on stage for next week’s Democratic debate, where his aides are betting he’ll showcase his strength and resiliency. Until then, though, Sanders has canceled his campaign events, emerging from his house for periodic walks with his wife Jane as the cameras roll.

But his campaign’s lack of transparency surrounding the episode has sparked a media backlash and reinvigorated worries about the trio of septuagenarians currently leading the race for the Democratic nomination, among which the 78-year-old Sanders is the oldest. His fellow White House hopefuls have faced similar calls to release their medical records, as did then-candidate Donald Trump during the 2016 race. Trump, who is 73, has shown a willingness to go after his rivals' perceived health problems, openly questioning his opponents' mental fortitude and fitness for office even as the president's critics have wielded the same accusations against him.

Sanders insisted Tuesday that he was “feeling good” and “getting stronger,” but dismissed concerns that his health would worsen his already existing campaign struggles.

Sanders admitted he’d erred in failing to listen to his body while on the campaign trail, but said he wants to turn the health scare into a teachable moment.

“I must confess that I was dumb,” he said, explaining that he felt lucky to have been born with “a lot of energy.”

“And during this campaign, I've been doing some cases three or four rallies a day running all over the state, Iowa, New Hampshire, wherever. And yet I, in the last month or two, just was more fatigued than I usually have been,” he continued. “I should have listened to those symptoms.”

He concluded: “So if there's any message that I hope we can get out there is that I want people to pay attention to the symptoms. When you’re hurting, when you’re fatigued, when you have pain in your chest, listen to it.”