US President Joe Biden announced Tuesday his bid "to finish the job" with re-election in 2024, plunging at the record age of 80 into a ferocious campaign that could set up a rematch against Donald Trump.

Launching his pitch in a video on the fourth anniversary of the day he first began his 2020 challenge against Trump, Biden said he was still fighting to save American democracy from Republican "extremists."

"When I ran for president four years ago, I said we're in a battle for the soul of America. And we still are," Biden pronounces in the voiceover.

"That's been the work of my first term: to fight for our democracy," Biden says. "Let's finish this job -- I know we can."

After a series of big domestic legislative wins, a strong economic recovery from the Covid pandemic, and momentous foreign policy struggles, Biden has no real challenger from within the Democratic Party.

But he will face constant and fierce scrutiny over his age.

The veteran Democrat would be 86 by the end of a second term. Even if a medical exam in February found him "fit" to execute the duties of the presidency, many including in his own voter base believe he is too old.

"I like what Biden's done. I think he's done a really good job," said retiree Roger Tilton, 72, as he walked near the White House. But "he's really too old for the job."

Biden likes to answer those concerns by saying, "watch me" -- meaning voters should focus on his policy wins at home and his marshaling of an unprecedented Western alliance to help Ukraine defend itself against Russia's invasion.

He made no reference to the age issue in his video, although a segment near the end showed him grinning and jogging into a public event.

- Republican infighting -
Over the next year and a half, Biden will have all the advantages of incumbency, backed by a united Democratic Party, while Republicans are only just starting their messy primary season.

Trump, despite becoming the first former or serving president to be criminally indicted -- and still facing probes into his attempt to overturn his loss to Biden in the 2020 election -- is the overwhelming Republican frontrunner.

The most likely Republican challenger to 76-year-old Trump, Florida Governor Ron DeSantis, presents a similarly right-wing figure, though starkly younger at 44.

Trump savaged what he called Biden's "calamitous and failed presidency."

"It is almost inconceivable that Biden would even think of running for reelection," he said in a statement on Monday.

The Republican Party posted a fictionalized video, purporting to show a dystopian future where Biden has won in 2024 and China invades Taiwan, the US southern border is overwhelmed and "officials closed the city of San Francisco" due to crime and drugs.

The US dollar, however, rose in trading after Biden's announcement.