US President Joe Biden faces one of the most critical moments of his political career Thursday with a State of the Union speech aimed at convincing skeptical voters the 81-year-old is fit to beat Donald Trump in November's election.

Most years the annual address to Congress is a largely ceremonial affair, but in 2024 the stakes are enormous as Democrat Biden seeks to overcome growing doubts about his age, the US economy and wars in Gaza and Ukraine.

Millions of Americans will also be tuning in to the primetime spectacle to see not just what the oldest president in US history says but how he says it, with Republican Trump, 77, continually mocking Biden's health and mental acuity.

Set for 9:00 pm (0100 GMT Friday), the speech is expected to trumpet achievements on infrastructure, drug prices and student debt, with Biden saying on Wednesday that he had done "more over three years than most presidents have in eight."

"You hired me to get the job done, build an economy that works for working people, and make life better for families," Biden wrote on X. "I'll update you on our progress and lay out the path ahead."

Trump and Biden nailed down a repeat of the 2020 election with their sweeping wins in this week's "Super Tuesday" primaries -- but it is a rematch that polls show many US voters do not actually want.

Biden is set to paint the election as an existential clash, warning that US democracy and its global standing would be at risk if Trump makes a sensational comeback to the White House.

Scandal-plagued and facing multiple criminal indictments -- including for trying to overturn his loss four years ago -- Trump has again appeared to defy the laws of political gravity thanks to his base of working-class, white voters.

'Stumbling'

But while Biden trails Trump in recent polls, he hopes his State of the Union speech will remind Americans why they voted for him four years ago and why they should give him a second term.

For three days over the weekend, Biden huddled with aides at the Camp David retreat crafting the speech, and he plans to "continue to fine tune" it on Thursday, White House Press Secretary Karine Jean-Pierre said.

Biden was already well prepared for a repeat of last year's heckling by hardline Trump allies during his State of the Union speech, the White House added -- but that is not the only risk.

Observers will also be on the lookout for signs of frailty after Biden suffered a series of trips and verbal slips in recent years.

"For every president, whether or not Biden, stumbling in some way, not necessarily physically, but over their words, that can often have an impact overshadowing what the president is saying," Sarada Peri, a former speechwriter for Barack Obama, told AFP.

Biden said his speech's key issues also included reproductive rights, which Democrats view as a major vote winner against Republicans.

The world will also be watching, with wars in Gaza and Ukraine meaning the stakes of the November 5 election could hardly be higher.

Biden is expected to urge Republicans to end the political paralysis in Washington that is blocking vital aid for Ukraine's fight against Russia's brutal invasion. 

The US president invited Ukrainian first lady Olena Zelenska and the widow of Russian opposition leader Alexei Navalny to be his guests at the US Capitol for the speech, but both were unable to attend, the White House said.

Trump has also pushed allies in Congress to blocking money to curb the migrant crisis on the Mexican border and military assistance for Israel's war against Hamas, saying they should not give Biden a win in an election year.