He is polling third behind Joe Biden and Elizabeth Warren in the race to take on President Donald Trump in 2020
US Senator Bernie Sanders may be 78 and recovering from a heart attack, but he is determined to prove he has lost none of his appetite for political battle ahead of elections next year.
The pugnacious left-winger is seeking to put his campaign for the Democratic nomination back on track after earlier suggesting that he would have to slow down his schedule due to his health scare.
"There's nobody who has run a more vigorous campaign," Sanders told ABC News in an interview broadcast on Sunday.
"We're talking about three or four rallies a day and town meetings... I think after a short period of time, we'll probably be able to return to that."
Sanders, who is polling third behind Joe Biden and Elizabeth Warren in the race to take on President Donald Trump in 2020, is renowned for his rigorous work ethic and reluctance to discuss personal matters.
But he offered a rare insight into how he felt before and during the heart attack -- which he brushed off as a minor issue.
"Heart attack is a scary word," he said. "What I had was a 45-50 minute procedure, two stents were placed in my heart, which had a blocked artery.
"It's a fairly common procedure, and people are back on their feet pretty soon, as is the case with me.
"I was more fatigued, despite a heavy schedule, than I should have been. I wasn't sleeping as well as I should have been.
"Occasionally, I was a little bit wobbly. And I should have put two and two together, and I didn't."
'Get back into the groove'
Sanders has pushed back after mentioning that he might reduce his workload.
"I misspoke the other day," he told NBC on Wednesday. "The media drives me a little bit nuts to make a big deal about it.
"We're going to get back into the groove of a very vigorous campaign, I love doing rallies and I love doing town meetings."
He also dismissed criticism that his team had been slow to release news of the heart attack, which he suffered on October 1 at a campaign event in Nevada.
"The first thing that we're trying to do is understand what's going on and not run to The New York Times and have to report every 15 minutes," he said.
"This is not a baseball game. So I think we acted absolutely appropriately."
He was released from the hospital after three days and is due to join rival candidates on Tuesday for a grueling TV debate before next weekend holding a "Bernie's Back" rally in New York.
Sanders is the oldest candidate vying to lead the Democrats. Biden is 76 and Warren is 70, while Trump is 73.
Sanders said he would "fight even harder" to introduce universal health care after his heart attack.
His health has generally been good and it has been Biden who has had to bat away questions about his stamina and mental sharpness.
A self-described Democratic socialist, Sanders laid out his radical personal vision to ABC, saying that "business as usual, and doing it the old-fashioned way, is not good enough."
"What we need is, in fact, I don't want to get people too nervous, we need a political revolution.
"I am, I believe, the only candidate who's going to say to the ruling class of this country, the corporate elite, enough with your greed and with your corruption. We need real change in this country."
Sanders pushed Hillary Clinton to the wire in 2016, but ultimately lost the race to become the Democratic nominee.