Joe Biden says he has decided to run for a second term and will formally announce his re-election campaign “relatively soon”.

“We’ll announce it relatively soon but the trip here just reinforced my sense of optimism about what can be done,” Biden told reporters at the tail-end of an emotional trip to Ireland.

“I told you my plan is to run again.”

Biden appeared in front of thousands of flag-waving well-wishers to a rousing Celtic punk anthem in an event that brought the kind of energy the Democratic president will hope to recreate in domestic rallies.

“The truth of it is, being here does feel … like coming home. It really does. Over the years stories of this place have become part of my soul,” Biden told the crowd in front of a cathedral that his great-great-great-grandfather helped build.

“This is a time of enormous possibility. And united by history, heritage and hope – and maybe most of all courage – nothing is beyond our reach.”

Biden has long hinted at his intention to contest the 2024 presidential race but the lack of a formal announcement has sometimes given jitters to supporters unsure if the 80-year-old Democrat, one of the oldest world leaders, would really commit to another four-year term.

‘Finish the job’

Yet behind the scenes, close aides and allies have already begun putting the steps in place to stand up a campaign infrastructure and fundraising apparatus ahead of a 2024 bid that could be a re-run of the 2020 match-up with Republican ex-president Donald Trump, who has already launched his campaign.

Biden’s expected pitch is to “finish the job” after a first term that included several legislative wins, including billions of dollars in federal funds to tackle the COVID-19 pandemic and for new infrastructure.

But Biden’s age makes his re-election bid a historic and risky gamble for the Democratic Party, which faces a tough election map to hold the Senate in 2024 and is now the minority in the House of Representatives.

His approval ratings are stuck in the low 40s; he would be 86 by the end of a prospective second term, nine years older than the average US male 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.

-AAP