Nancy Pelosi, the former Speaker of the US House of Representatives, says she will seek re-election next November.
Mrs Pelosi, 83, was first elected to her San Francisco district in 1987, before serving two terms as speaker between 2007-2011 and 2019-2023.
She led House Democrats for two decades before standing down as leader after Republicans won last year's election.
Her decision to run again is sure to reignite debate about the age of US political leaders.
"Now more than ever our City needs us to advance San Francisco values and further our recovery," Ms Pelosi wrote in a post on X, formerly known as Twitter.
"Our country needs America to show the world that our flag is still there, with liberty and justice for ALL. That is why I am running for re-election - and respectfully ask for your vote."
Mrs Pelosi is the first woman in US history to serve as speaker of the House and has played a critical role in advancing - or thwarting - the agendas of multiple presidents.
She is widely credited with marshalling the passage of former President Barack Obama's signature healthcare legislation, as well as bills to address infrastructure and climate change under incumbent President Joe Biden.
Mrs Pelosi also directly challenged Donald Trump throughout his presidency, famously ripping up a copy of his State of the Union address behind his back.
She stood down as Democratic leader after the Republicans narrowly re-took the House in last year's election and Kevin McCarthy was elected speaker after a lengthy process.
That decision to quit the top job had fuelled speculation that she would retire after 2024's midterm election.
But US media reported that she told top advisers and activists at a breakfast on Friday of her plans to seek re-election, before formally announcing the move on X.
Her decision to seek a 19th term in office comes as the country reflects on the rising age profile of its top politicians.
US President Joe Biden will be 81 by the time he seeks re-election next year, while his most likely opponent - former President Donald Trump - will be 78.
Meanwhile, many questioned whether the top Republican in the Senate, Mitch McConnell, ought to stand down after the 81-year-old appeared to freeze for the second time in just over a month at a media event last week.
Polling consistently shows a majority of Americans - about three-quarters - favour age limits on people who serve in the White House and Congress.
And roughly the same proportion say they are concerned that Mr Biden's age could affect his physical and mental competence, according to CNN.