Joe Biden wanted to save the "soul of America" from Donald Trump, but his stubborn defiance of the march of time threatened to clear his rival's path back to power.

Biden caved in to growing pressure Sunday and announced he is dropping out of the presidential race, amid concerns over his mental acuity and ability to beat Trump and serve four more years.

His decision saw him hailed as a "patriot of the highest order" by his one-time boss, former president Barack Obama. But it was an agonizing road to get there as Biden fought for his political survival after a disastrous debate, repeatedly citing his family's mantra that "when you get knocked down, get back up again."

From playground punch-ups to a stutter to terrible family tragedies, the 81-year-old Democrat had long seen his life story as a series of comebacks against impossible odds.

And it was his triumph four years ago against Trump that convinced Biden that, despite being the oldest president in US history, he was the only one who could do it again -- until he threw in the towel on Sunday.

Overcoming his reputation as a gaffe machine, Biden initially lived up to his goal as a "unifier in chief" after the Trump years and the shock of the January 6, 2021, attack on the US Capitol.

But the question of his age always loomed large.

Biden eventually joked about it but always denied it was an issue, even after a debate where his rambling answers and listless stare sparked a revolt by Democrats.

A mix of pride and his conviction that Trump was a threat to democracy kept Biden fighting down to the wire.

Franklin Foer, author of a book on the Biden presidency, wrote recently 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," he wrote in the Atlantic.

That outlook was largely formed by a hardscrabble childhood in Scranton, Pennsylvania, in the American rust-belt. Biden was part of a close-knit Irish Catholic family -- he was just the second Catholic US president after his hero John F. Kennedy -- that was 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.

"He decided to fight... BANGO -- he'd punch the guy in the face," Cramer wrote.

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 won an unlikely victory to be elected senator for Delaware when his wife Neilia and their one-year-old daughter Naomi were killed in a car crash. 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 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."

With his family close around him, Biden did not let two failed presidential bids -- and a nearly fatal aneurysm in 1988 -- discourage him.

He served as Obama's vice president for two terms, and his stubborn persistence in pursuit of the top job paid off when he came out of retirement to beat Trump in 2020, defying critics who said he was too old.

Saying at his inauguration he wanted to be a "president for all Americans," his old-fashioned centrism was a relief to many after the divisiveness of the Trump years.

At home he forced through a massive Covid recovery scheme and a green investment plan.

In Kamala Harris, his likely successor to the Democratic nomination, he appointed the first female, Black and South Asian vice president in US history. She said Sunday she would work to "earn and win this nomination."

US allies welcomed Biden's pledge that "America is back" and his strong support for Ukraine.

But despite the best efforts of the White House to limit his public appearances, his age became the story again. A series of senior moments culminated in the disastrous debate performance against Trump that doomed his bid for a second term.

As he fought to save it, he returned to the image of the underdog, often repeating his father's saying: "Don't compare me to the Almighty, compare me to the alternative."

Now Democrats have done just that. (AFP)