Atlanta (AFP) - Joe Biden and Donald Trump square off Thursday in a US presidential debate that carries enormous stakes for both candidates as they battle for any advantage in their tight race for the White House.

Millions of Americans will tune in for a clash taking place months earlier than usual – the opening salvo in what promises to be a bruising summer of campaigning, in a deeply polarized country still scarred by the chaos and violence of the 2020 election.

With only two debates this election cycle and national polls showing the pair locked in the tightest of contests, Thursday’s event at CNN headquarters in Atlanta – starting at 9:00 pm (0100 GMT Friday) – takes on heightened significance.

For many voters, the choice between the 81-year-old Biden, the oldest ever incumbent, and the 78-year-old Trump, now a convicted felon, is an uninspiring one – a dynamic both candidates need to counter.

For Trump, an instinctive political pugilist, the challenge will be to rein in the aggressive tendencies he unleashed – to his detriment – in their first chaotic debate four years ago.

The Democratic president will be desperate to avoid any major gaffes that could underline concerns about his age.

He also will focus on his key campaign message that Trump poses an existential threat to US democracy.

Biden arrived in Georgia about six hours before the on-air showdown, and took the opportunity to greet supporters outside his hotel in Atlanta.

His challenger touched down later aboard his private jet, pumping his fist to supporters on the tarmac before he ducked into a waiting vehicle.

There is certainly no love lost between the two candidates, and both sides have agreed to debate rules aimed at minimizing the prospect of a shouting match.

There will be no studio audience, depriving them of the momentum that comes from ginning up supporters, and microphones will cut out when a candidate’s speaking time is over.

- Pre-debate attacks -

To coincide with the debate, both camps unleashed fresh attack ads, with Trump’s team taking out TV spots accusing Biden of mismanaging the economy and encouraging illegal immigration.

In a morning post on his Truth Social platform, Trump argued that Biden was the real threat to democracy, “and a threat to the survival and existence of our country itself.”

The Democratic Party, meanwhile, paid for five massive billboards in Atlanta trolling Trump over his legal woes.

“Donald, welcome to Atlanta for the first time since becoming a convicted felon. Congrats – or whatever…” the signs said.

Trump enjoys a slight advantage in the all-important swing states, but polling is close in an election likely to be decided in a handful of battlegrounds.

The latest Quinnipiac University poll 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 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 debate, both Trump and Biden have had missteps, 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, and fears he would weaponize the presidency to settle personal scores.

- Fine-tuning or freewheeling -

Biden has spent the last 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, with informal policy roundtables and workshopping strategy with rally crowds.

“Biden’s a good debater. He’s been in a lot of these debates over many, many decades so one should not underestimate him,” California Governor Gavin Newsom, a Democrat, told reporters in a “spin room” before the debate.

Trump advisor Jason Miller in turn said that “many in the media are already prepared to give Joe Biden a participation trophy if he can simply stand upright for 90 minutes.”

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