Joe Biden and Donald Trump square off for a historic US presidential debate Thursday, with the stage set for what could be an inflection point in the 2024 race as millions of voters tune in to the clash of rivals.

The showdown -- between the oldest-ever incumbent and a convicted felon -- fires the starting gun on what promises to be a bruising summer of campaigning in a deeply polarized and tense United States still convulsed over the chaos and violence that accompanied the 2020 election.

With only two debates this election cycle, Thursday's high-stakes event takes on heightened significance, and both candidates have stepped up their personal attacks, with national polls showing the pair neck and neck.

As the candidates applied finishing touches to their debate strategy, one critical question remained: Will Trump the political pugilist rein in his aggressive tendencies or unleash his unruliness, much as he did in their first chaotic debate four years ago? 

"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 president will be desperate to avoid any major gaffes. He meanwhile was pulling no punches Wednesday -- at least on social media, where he said "Donald Trump is the greatest threat to our democracy," a message he will almost certainly seek to hammer home on debate night.

The president arrives Thursday on Air Force One in Atlanta, capital of southeastern battleground Georgia, for the duel at CNN headquarters, while his challenger flies in on his private jet, nicknamed "Trump Force One."

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 debate 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, his recent conviction on 34 counts of falsifying business records, not to mention other criminal cases he faces, and fears he would weaponize the presidency to settle personal scores.

Fine-tuning or freewheeling

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

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 told 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.

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 since the president's new executive action last month cracking down on the border.

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 the debate would change their vote.