ATLANTA, United States (AFP) — Joe Biden and Donald Trump square off Thursday in a crucial 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 to the highly anticipated clash — the opening salvo in what promises to be a bruising summer of campaigning, in a deeply polarised 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 a series of debate rules aimed at minimising 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.

To coincide with the debate, both camps unleashed a series of 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.