President Joe Biden launched his re-election bid on Tuesday with a promise to protect American liberties from "extremists" linked to former president Donald Trump, whom he beat in 2020 and might face again in 2024.

Biden made his announcement in a video released by his new campaign team that opens with imagery from the Jan 6, 2021, attack on the US Capitol by Trump's supporters.

"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 said.

"This is not a time to be complacent. That's why I'm running for re-election.

"Let's finish this job. I know we can," he said.

He described Republican platforms as threats to American freedoms, vowed to fight efforts to limit women's healthcare, cut Social Security and ban books, and blasted "MAGA extremists".

MAGA is the acronym for the "Make America Great Again" slogan of Trump, who is the early frontrunner in the Republican primary race. If he wins, he will face off against Biden again in the November 2024 election.

Biden, 80, must overcome Americans' concerns about his age in order to win re-election, with 44 percent of Democrats saying he is too old to run, a Reuters/Ipsos poll completed on Monday found.

Trump, 76, also faces concerns about his age, with 35 percent of Republicans saying he is too old.

The poll showed that a majority of registered voters don't want either Biden or Trump to run again.

While Biden's approval rating is relatively low, his aides are confident he can beat Trump again. The Reuters/Ipsos poll showed him with a lead of 43 percent to 38 percent over his Republican rival among registered voters.

In his campaign video, Biden squarely targeted Trump and his allies.

"Around the country, MAGA extremists are lining up to take on those bedrock freedoms, cutting Social Security that you paid for your entire life, while cutting taxes for the very wealthy, dictating what healthcare decisions women can make, banning books, and tell people who they can love, all while making it more difficult for you to be able to vote," Biden said.

In the two years since he took over from Trump, Biden won Congress’ approval for billions of dollars in federal funds to tackle the Covid-19 pandemic and for new infrastructure, and oversaw the lowest levels of unemployment since 1969, although 40-year inflation highs have marred his economic record.

Speaking to a meeting of North America's Building Trades Unions on Tuesday, Biden said his economic plan was working but there is "more to do". Biden listed his policy achievements and the crowd chanted "four more years!"

Biden's age makes his re-election bid a historic and risky gamble for the Democratic Party, especially if he faces a much younger Republican candidate.

Democrats already face a tough election map to hold the Senate in 2024 and are the minority in the House of Representatives now.

Biden would be 86 by the end of a prospective second term, almost a decade higher than the average US male's life expectancy.

Doctors declared Biden, who does not drink alcohol and exercises five times a week, "fit for duty" after an examination in February. The White House says his record shows that he is mentally sharp enough for the rigours of the job.

Biden will be joined in his 2024 quest by his running mate, Vice-President Kamala Harris, who is featured prominently in his campaign video.

Trump matchup again?

In a statement about Biden's candidacy, Trump criticised the president over his record on immigration, inflation, and the chaotic US pullout from Afghanistan in 2021.

"American families are being decimated by the worst inflation in half a century. Banks are failing," Trump said on his social media platform.

"We have surrendered our energy independence, just like we surrendered in Afghanistan," he said.

Marking a sharp contrast to Biden's campaign announcement, Trump is on trial in a civil lawsuit this week over writer E Jean Carroll's accusation that he raped her in a department store dressing room in the mid-1990s.

The former president, who is not required to attend the trial, has denied raping Carroll.

Biden is unlikely to face much competition from inside his party. No senior Democrats have shown signs of challenging him.

Potential and declared Republican presidential candidates have begun framing the 2024 election around cutting back government spending amid still-high inflation, restricting abortion, crime in Democratic-run cities and illegal immigration.

The two leading Republican contenders, Trump and Florida Governor Ron DeSantis, want to limit the access of transgender children to sports teams and gender-affirming medical care, and restrict how schools teach LGBTQ+ issues and America's history of race.

During a briefing with reporters, White House spokesperson Karine Jean-Pierre at first declined to answer a question about whether Biden would serve out another four-year term if re-elected.

"I wanted to be sure that I didn't go into 2024 more than is appropriate under the law," she wrote later on Twitter.

"But I can confirm that if re-elected, (Biden) would serve all eight years."

Biden ran a mostly virtual campaign to defeat Trump in the 2020 election as Covid-19 raged.

With pandemic restrictions mostly over in the United States, the 2024 race is likely to be a much different, more physical affair.

After losing to Biden in 2020, Trump refused to concede defeat, falsely claiming that there had been widespread electoral fraud.

His supporters stormed the US Capitol building in Washington on Jan 6, 2021, in support of his claims but they failed to halt certification by Congress of Biden's win.

Biden's campaign video suggests he plans to regularly remind voters of those events between now and the next election.

Other Biden themes may include strong US support for Ukraine in its war against Russia and what the White House says are Republican plans to unravel federal healthcare.

- Reuters