Joe Biden and Donald Trump made final preparations 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 televised showdown will raise the campaigning to boiling point, with tens of millions expected to tune in and both camps 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.

Trump enjoys a slight advantage in the all-important swing states but the overall polling looks extremely close in an election likely to be decided by a few photo finishes in a handful of battlegrounds.

The latest Quinnipiac University poll conducted Sunday shows Trump edging ahead of Biden nationally, 49 percent to 45 percent.

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

Biden, 81, faces the most concern about his mental sharpness, with voters much more likely to bring up his age than Trump's, despite the Republican being just three years younger.

Ahead of the first ever debate between two candidates who have already served in the Oval Office, both Trump and Biden have had missteps that have raised questions over their age, occasionally stumbling over words or appearing muddled.

Trump is also engulfed in controversy over his inflammatory rhetoric and a glut of criminal cases he faces, as well as fears that he would weaponize the presidency to settle personal scores.

'Out for himself'

Biden has spent the week off the radar at the mountainside retreat of Camp David near Washington, fine-tuning his attack lines in a series of mock debates under real TV studio lighting.

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.

"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 Democratic National Committee 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.

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

Trump and his team have also been pushing the baseless theory that Biden will be hyped up on performance-enhancing drugs and have made repeated insinuations of bias from 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.

'Stakes'

One of Biden's biggest vulnerabilities is border security, with Trump promising to combat an influx of undocumented migrants from Mexico with mass deportations and repeatedly bringing up killings by migrants.

The Biden administration said Wednesday there has been a 40 percent drop in illegal crossings from Mexico since the president's new executive action last month cracking down on the border.

Two-thirds of registered voters said in a new YouGov poll they would probably or definitely watch the debate, with Biden and Trump supporters equally likely to tune in.

More Americans expect a Trump debate win than a Biden victory -- 40 percent to 30 percent -- but just one in 10 thought it even somewhat likely that the debate would change their vote.

Former lawmaker Adam Kinzinger joined a small group of fellow Republican Trump critics at the Georgia state capitol in Atlanta to rally support for Biden ahead of the debate.

"If you had told me, Adam Kinzinger of three years ago, that 'you're going to be endorsing a Democrat for president in three years,' I probably wouldn't have believed you," he said.

"But I've got to tell you the stakes of the moment are way too high."