US President Joe Biden has announced he will run for re-election in 2024, setting the stage for a potential rematch with Donald Trump.
The Democrat had been expected to seek a second four-year term and launched his campaign in a video on Tuesday.
He said it was a pivotal moment with freedoms and rights under threat. "This is not a time to be complacent," he said. "That's why I'm running".
Vice-President Kamala Harris, 58, will once again be his running mate.
Mr Biden, 80, is already the oldest president in US history and is likely to face questions over his age throughout the campaign. He would be 86 after finishing a second full term in 2029.
"It's legitimate for people to raise issues about my age," he said earlier this year. "And the only thing I can say is, watch me."
Mr Biden faced off against Mr Trump in 2020, defeating the Republican after promising to "restore the soul of the United States".
"When I ran for president four years ago, I said we are in a battle for the soul of America - and we still are," Mr Biden said in the three-minute announcement video, which shows the president meeting a diverse range of Americans.
Mr Trump has already launched his bid for the presidency, raising the prospect that both men will face each other once again on 5 November 2024. Both are considered favourites to win their nominations.
For months now, Mr Biden has made clear that he will stand for re-election with the main question being when he would announce. After spending the weekend with aides at Camp David, the presidential retreat in Maryland, he opted to launch his campaign on the fourth anniversary of his 2020 announcement.
Julie Chavez Rodriguez, a senior White House adviser, will serve as his campaign manager.
As yet Mr Biden has no major challengers for the Democratic nomination meaning a smooth path to the candidacy is almost certain.
But recent polls suggest his decision to run is a divisive one both within the party and nationally. An NBC News poll over the weekend found that 70% of Americans, and just over half of Democrats, believe he should not run again.
A majority of people who said Mr Biden should not run cited his age as a concern. Forty-eight percent said it was a "major concern".
His approval ratings remain negative by a significant margin but have recovered slightly from their lowest point in mid-2022, when 36% of Americans said they approved of his performance as president.
In comparison, Mr Trump experienced similarly low approval ratings while in office. In December 2017, 33% of respondents approved of the job he was doing.
Despite these polls, Mr Biden's hopes of re-election were boosted late last year when his party performed better than expected in the midterm elections.
He also has a series of legislative achievements to tout on the campaign trail, including a $1.2tn infrastructure bill and the marshalling of Western support for Ukraine since Russia's invasion.
There are currently two other announced candidates for the Democratic nomination - best-selling self-help author Marianne Williamson and anti-vaccine activist Robert Kennedy Jr.
The lack of any formidable rivals in his party allowed Mr Biden to set the timing of his announcement without significant external pressure.
His advisers have said he sees an advantage in drawing a contrast between his role governing the nation while his potential Republican opponents engage in partisan campaigning or - in Mr Trump's case - deal with the fallout from criminal investigations.
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