U.S. President Joe Biden on Wednesday explained to the American people his exceedingly rare decision to bow out of the race for re-election, saying he set aside his personal aspirations based on the belief that "the defence of democracy is more important than any title."

Biden, 81, said he believed he deserved a second term at the White House based on his leadership record, but he stepped aside to allow the Democratic Party to come together in support of the next generation after a tumultuous time in its history.

"In recent weeks, it's become clear to me that I need to unite my party in this critical endeavour," Biden said, speaking in the fourth Oval Office address of his presidency.

"Nothing — nothing — can come in the way of saving our democracy. That includes personal ambition. I've decided the best way forward is to pass the torch to a new generation.

"I revere this office, but I love my country more."

The 11-minute address carried the emotional heft of a concession speech but largely focused on the future, as Biden laid out his plans for the remainder of his term and threw his support behind the candidacy of Vice-President Kamala Harris. Political scientists said he focused on those two themes to cement his own legacy as a five-decade political veteran and make way for the next leader of the party — a fitting coda for a man whose sense of duty runs a lifetime deep.

"His speech to me was very similar to the speech that a two-term, eight-year president would make — wrapping up what they've done, what they've accomplished and what they think is to come next," said Georgia Kernell, an assistant political science professor at the University of California, Los Angeles.

"I think in many ways, it was a a way to package Biden as finishing his term on his terms, instead of because he was under so much pressure."

WATCH | Biden on making way for the next generation of leadership:

Time to 'pass the torch,' Biden says

U.S. President Joe Biden said it was time to 'pass the torch' to a new generation on Wednesday, explaining his decision to drop out of the presidential race in an address from the Oval Office.

The speech was the first chance for the public to hear directly from Biden his rationale for leaving the presidential race, days after he made his announcement Sunday afternoon following weeks of insistence that he was still the best candidate to defeat Republican Donald Trump on Nov. 5. 

Biden's candidacy fell apart after a disastrous debate performance last month, where he spoke haltingly and failed to counter a vigorous opponent, even as Trump at times spoke with little concern for verifiable facts. Having already long suffered doubts about his age and acuity, Biden could not overcome the public perception and party panic that he was no longer fit for the nation's highest office.

He conceded on Wednesday that it was time to make way for "new voices, fresh voices — yes, younger voices," an about-face from his stance just weeks ago, when he told ABC News that only the "Lord Almighty" could force him from the race.

It was a monumental moment for a lifetime politician who is known to take quitting deeply seriously.

In the hours before he gave up his first presidential bid in 1978, Biden paced his living room and looked for a way to save a campaign suffering death by plagiarism scandal.

"I've never been a quitter ... never quit anything in my life," Biden said, according to Richard Ben Cramer's Pulitzer-winning account of the 1988 campaign, What It Takes: The Way to the White House.

Decades later, as he accepted the vice-presidential nomination at the 2008 Democratic National Convention, Biden said, "Failure at some point in your life is inevitable, but giving up is unforgivable."

WATCH | Canadian politicians salute Biden, look ahead to Harris:

Canadian politicians salute Biden, look ahead to Harris

Ontario Premier Doug Ford, NDP Leader Jagmeet Singh and Minister of Industry François-Philippe Champagne are among the Canadian political leaders saluting U.S. President Joe Biden and looking forward to potentially working with Kamala Harris on maintaining close and co-operative ties between the two countries.

In his 2017 memoir, which wove details of his vice-presidency with deliberations over another presidential run, Biden recounted how public life had taken on new meaning after his son Beau told his father he was obligated to take another run at the White House.

"Giving up on the presidential race would be like saying we were giving up on Beau," Biden wrote in Promise Me, Dad.

Beau Biden, who had brain cancer, died in May 2015. His father didn't mount a campaign the following year but won the presidency in 2020.

Biden announced on Sunday that he would give up his run for a second term and back Harris in a letter posted on X, formerly Twitter, as he recovered from COVID-19 at his beach house in Rehoboth Beach, Del. In the three days since, an invigorated Democratic Party has rallied around Harris, rushing to grant endorsements and raising tens of millions in campaign donations.

Biden praised Harris as "experienced, tough [and] capable" in his address on Wednesday. UCLA's Kernell said he likely didn't spend more time praising the vice-president because she's already riding her own momentum.

"Had it not looked like that, he might have then had more of an incentive and desire to make a case for [Harris]. But I think that, as it was, she's doing fine on her own," she said.

Biden has called Trump an existential threat to the nation's democracy. He did not mention the former president by name in his address but said American voters heading to the polls needed to choose between "moving forward or backward."

"The great thing about America is here, kings and dictators do not rule. The people do. History is in your hands. The power is in your hands. The idea of America lies in your hands," he said.

Melissa Haussman, a political science professor at Carleton University in Ottawa, said Biden's address was also an appeal to voters.

"There have been a lot of accomplishments, but it was a short, brief speech to really lay the ground for what's at stake," she said.

"Tonight was just signaling that yes, he's out, he wants the party united — which, frankly, has been a bit of a heavy lift at times — and to tell everybody to really get with the program."

First Lady Jill Biden, daughter Ashley and several senior advisers sat off-camera along the curved Oval Office wall as Biden delivered his remarks.

Biden said he will spend his last six months in office focusing on his presidential duties. He said he would help strengthen NATO, stop Russian President Vladimir Putin from taking over Ukraine, pursue a ceasefire agreement in Gaza and push for Supreme Court reform. He is scheduled to meet Israeli Prime Minister Benjamin Netanyahu on Thursday to discuss ceasefire negotiations.

Earlier Wednesday, White House press secretary Karine Jean-Pierre said any question of Biden resigning his office — which would allow Harris to run as an incumbent — was "ridiculous."

Jean-Pierre said Biden has "no regrets" about his decision to stay in the race as long as he did, or his decision to quit it over the weekend.

Biden is the first sitting American president to abandon a bid for a second term since Lyndon B. Johnson, who, facing mounting criticism for his stance on the Vietnam War, stepped away from the election in 1968. Although not unprecedented, Biden's choice was rare because it came with just 107 days left until election day.