Anyone expecting Joe Biden to step aside from the US presidential race without a fight had not fully considered the life story of a proud but often stubborn man.

From playground punch-ups to terrible tragedies and multiple White House bids, Biden has long seen his life as a series of comebacks against impossible odds.

And as a Democratic revolt over his debate debacle against Donald Trump appears to sputter for now, the 81-year-old seems determined to win the fight of his political life.

Unless a major shift occurs, it will likely fall to US voters to decide whether Biden stages another turnup for the books -- or whether hubris condemns him and his party to a historic defeat by Trump.

Biden has repeatedly returned to the image of himself as an underdog since the debate, repeating his family's mantra that "when you get knocked down, you get back up."

"What we've seen the last 10 to 12 days is certainly fundamental to the Joe Biden story," his spokeswoman Karine Jean-Pierre said from the White House podium on Tuesday.

"He is someone that is certainly counted out many, many times in his career. People tend to knock him down, and you hear him say he gets back up.

"This is the story of him standing up for himself, standing up for millions of Americans."

- 'Punch the guy' -

That outlook was formed by a hardscrabble childhood in the American rust-belt, as part of a close-knit Irish Catholic family known for its intense pride.

His mother Jean told the young Joey and his siblings every day that "nobody was better than a Biden," Ben Cramer wrote in his book "What It Takes," about the 1988 US election campaign.

He was also known for never backing down.

"Most guys who got into a fight, they'd square off... Joey didn't do that," Cramer wrote. "He decided to fight... BANGO -- he'd punch the guy in the face."

One affliction Biden famously had to battle was a childhood stutter.

Repeatedly humiliated at school, the young Biden ended up teaching himself how to speak smoothly by sheer determination, repeating phrases again and again into the mirror.

But Biden's biggest test was yet to come.

In 1972, he was only 29 and had just been elected senator for Delaware when his wife Neilia and their one-year-old daughter Naomi were killed in a car crash, while their young sons Beau and Hunter were left badly injured.

Tragedy struck again in 2015 when Beau died of brain cancer aged 46.

Biden also had to deal with the agony of Hunter's severe drug addiction and legal problems.

"Sometimes I marvel at Joe's strength. His life has been marked by cruel losses," First Lady Jill Biden, whom Biden married in 1977, said in her memoir "Where the Light Enters."

- 'Psychological prison' -

With his family close around him, Biden has also ridden out a series of political humiliations.

In 1988 he was forced to abandon his first presidential bid following a plagiarism scandal.

His next bid in 2008 ended in heavy defeat in the Democratic primaries, before Barack Obama picked him as his running mate.

Yet in the current crisis over Biden's age and health, the very things that have previously brought Biden strength could also bring about his downfall.

It's common knowledge that he will only really listen to family members and a few aides he's known for decades, but as he ages that bubble has become increasingly insular.

His long-standing belief that he's been underestimated and mocked by the media means he's even less likely to listen to outside voices.

Moreover, Biden's lifelong image as someone who always bounces back means he may not be able to envisage a graceful exit this time around.

Franklin Foer, author of a book on the early Biden presidency, wrote recently in The Atlantic magazine that "humiliation -- and its transcendence -- is Biden's origin story."

"Right now it is his psychological prison, a mental habit that might doom American democracy."