Democratic presidential candidate and former U.S. Vice president Joe Biden speaks at the Teamsters Vote 2020 Presidential Candidate Forum on 7 December 2019 in Cedar Rapids, Iowa. (AFP)

He’s supposedly too old, too moderate, too prone to campaign stumbles.

But despite the criticism long circulating about Joe Biden, he has proven the most resilient candidate in the Democratic presidential nomination race — at least for now.

The 77-year-old former US vice president is capping off a big week that has seen him take on a more aggressive tone on the campaign trail.

He was feeling so invigorated and pugnacious during his eight-day “No Malarkey” bus tour through early-voting Iowa that he challenged an obstinate voter to a push-up contest Thursday.

The elder statesman is the national frontrunner in the race to challenge President Donald Trump in next November’s election.

As Barack Obama’s deputy for eight years, Biden today has a commanding lead among African Americans, a critical constituency in the Democratic nomination battle.

His blue-collar roots and appeal is also seen as a selling point to working-class white voters, many of whom may be frustrated with Trump and are looking for a viable alternative.

Biden’s numbers have slipped in Iowa and New Hampshire, the two states that vote first in the nomination race, where he trails rivals Bernie Sanders, Elizabeth Warren and Pete Buttigieg.

But he has swatted away threats posed by other challengers, including Senator Warren, whose bright rise has deflated amid skepticism over her universal health care plan.

Biden’s new ad branding Trump a global laughing stock was well-received, and footage of Biden confronting that Iowa voter went viral, giving millions a glimpse of the candidate showing fire in the belly.

When the elderly voter, a retired farmer, accused Biden of helping his son Hunter Biden get a spot on the board of a Ukrainian gas company while his father was vice president, the candidate snapped.

“You’re a damn liar, man,” Biden shot back.

Also this week Biden landed a key endorsement: former Democratic presidential nominee John Kerry.

Biden “helped President Obama make some of the toughest and loneliest decisions of a president,” former secretary of state Kerry told voters at a Biden event Friday in Cedar Rapids.

“That’s what this is about. Seasoning. Experience.”

Polls show Democrats want an electable candidate who can go toe to toe against the current president.

“Democrats have a burning desire to defeat Trump,” Larry Sabato, director of the Center for Politics at University of Virginia, told AFP.

“Most Democrats think Biden has the best chance of doing that — although it’s not a slam dunk.”

Indeed less than two months before Iowa votes there is uncertainty about the Democratic field, including Biden.

His debate performances have been hit or miss. He has struggled to raise money, trailing rivals like Buttigieg and Sanders in campaign contributions.

And billionaire Mike Bloomberg, the former mayor of New York, likely would have stayed out of the race if he believed fellow centrist Biden was a strong candidate.

But Biden has exhibited staying power at least through 2019, and Democratic strategists have looked on with a mixture of respect and bewilderment.

“The Biden thing is the strangest thing I’ve ever seen in politics because the guy is up there in the air and everybody is just assuming he’s going to come down,” Obama’s campaign guru David Axelrod told Politico recently.

“But he’s still driving, you know? He’s still moving forward.”

Biden’s campaign has been beset with problems, but it keeps bouncing back.

During the first Democratic debate in June, Biden got attacked by Senator Kamala Harris for his previous positions on busing and his civil rights record.

Biden was shaken. But he recovered, and now Harris, the only African-American woman in the field, has dropped out.

Warren, who surged into a tie with Biden in early October, has seen her campaign lose nearly half its support in the two months since, while Biden’s national lead has widened slightly, according to a RealClearPolitics.com average of polls.

Former Obama cabinet member Julian Castro went after Biden hard in one of the debates — and Castro’s longshot campaign has flatlined.

Biden may yet stumble, or sink from attacks by Republicans who see him as Trump’s chief 2020 nemesis.

But for now the Democratic frontrunner chugs along, despite party handwringing.

“Biden is not a cinch to win,” Sabato said. “But at the moment you’d rather be Biden than anybody else.”