Atlanta, Georgia - Joe Biden and Donald Trump prepared Wednesday for the biggest moment so far in the US election – the first of two high-stakes debates that could upend the race.

Thursday's showdown will raise the campaigning to boiling point, with both camps recently escalating their increasingly personal attacks.

"I think I have been preparing for it for my whole life...We'll do very well," Trump told right-wing network Newsmax in an interview on his debate preparation.

The 2024 election looks close, with Trump enjoying a slight polling advantage in the all-important swing states in an election likely to be decided by a few hundred thousand votes across a handful of battlegrounds.

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The rivals both step onstage for the 90-minute clash, hosted by CNN in the southern city of Atlanta, seeking to allay fears about serious political liabilities.

Biden spent the week off the radar at the mountainside retreat of Camp David near Washington, preparing with mock debates. Trump's preparation has been more relaxed, eschewing dress rehearsals in favor of informal policy roundtables and workshopping debate strategy with rally crowds.

Aides have encouraged him to focus on his perceived strength on the economy and crime, while Biden will seek to paint Trump as unhinged and unfit for office.

The Biden campaign released an ad saying Trump is preoccupied with "revenge" rather than helping voters, and the Democratic National Committee put up billboards in Atlanta reminding voters that the Republican is a convicted felon.

"Come November, Georgians will head to the polls remembering that President Biden looks out for them, while white-collar crook Donald Trump will only look out for himself," said DNC spokesperson Jackie Bush.

The Trump campaign has repeatedly characterized Biden as feeble and incompetent but changed tack in recent days following warnings that setting low expectations for the Democratic president would only help him.

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"We know that Joe Biden, that after taking an entire week off, will be ready for this," senior Trump campaign advisor Jason Miller told reporters.

Backed by right-wing media figures, Trump and his team have been pushing the baseless theory that Biden will be hyped up on performance-enhancing drugs.

The ex-president and his people have also been accused of "working the ref" and laying the groundwork for a poor Trump performance by repeatedly making insinuations of bias on the part of CNN.

Miller said Trump would emerge as the clear choice if "allowed" to set out his vision for America "without the blatant interference of CNN or the two moderators" of the debate – the first ever between two candidates who have already served in the Oval Office.