(AFP) - US President Joe Biden announced Tuesday he will seek re-election in 2024 and "finish the job," 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 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 pronounced in the voiceover.

"That's been the work of my first term: to fight for our democracy," Biden said. "Let's finish this job."

After a series of big domestic legislative wins and momentous foreign policy struggles, including leadership of the Western coalition helping Ukraine resist Russian invasion, Biden has no real challenger from within the Democratic Party.

Another big boost is a powerful post-pandemic US economic recovery, helped by historic federal spending to renew infrastructure and encourage investment in the high-tech electric vehicle and semiconductor sectors.

"That's what we're doing: rebuilding America," Biden told a crowd of cheering trade union workers at a Washington hotel conference room later Tuesday, in his first speech since launching his re-election campaign.

Biden cast himself as champion of blue collar Americans and said Republicans cared more about Wall Street.

"I think there should be a minimum tax for billionaires," he said in the speech, interrupted by extended applause and a chant of "four more years!"

"No billionaire should be paying a lower tax rate than a construction worker, a school teacher, a firefighter, a cop, a nurse. I mean it, it’s simply wrong," he said.

Biden was fired up, but he will face constant and fierce scrutiny over his age.

He 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."

- 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.

His precarious legal position was underlined Tuesday when jury selection began in yet another case -- a New York civil trial over an allegation of rape.

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."

The Republican Party posted a AI-generated 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, immediately rose in trading after Biden's announcement.

- 'Better than the alternative' -

The first word in Biden's video, following eerie footage of the attack by Trump's supporters against Congress on January 6, 2021, was "freedom."

That set the tone for the first half of the message, before a shift to Biden's trademark optimism and his pronouncement that "there is nothing, simply nothing, we can't do if we do it together."

A montage of images in factories and construction sites underlined Biden's argument during his first term that he has been restoring the country's manufacturing base and creating jobs for the middle class.

Meanwhile, Vice President Kamala Harris -- staying with Biden on the 2024 ticket -- was to speak on another big vote winner for Democrats: abortion rights in an era of growing Republican-led restrictions.

Biden's approval ratings have not topped 50 percent for more than a year and a half.

However, he has consistently over-delivered when it matters. Supporters say the Democratic Party's surprisingly strong performance in 2022 midterm congressional elections validated the Biden brand.

And while Biden may seem bland in comparison to Trump, he is banking on his moderate, old-fashioned image being the secret weapon needed in an increasingly extreme era.

"I think when folks look at President Biden and his strong record compared to the alternative, they will vote for him, and the polls show that," Democratic Senator Chris Coons told CNN.

© Agence France-Presse