Atlanta, Georgia - 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 the approval of undecided voters and any slim advantage in their neck and neck race for the White House.

Millions of Americans will tune in to the highly anticipated clash that fires the starting gun on what promises to be a bruising summer of campaigning in a deeply polarized country still scarred by the chaos and violence that accompanied 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, Georgia, takes on heightened significance.

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.

Donald Trump Biden-Trump debate lowlights: The worst moments from a bad night

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 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 shows Trump edging ahead of Biden nationally, 49% to 45%.

As Biden and Trump take the stage at 9 PM ET, the other presidential hopefuls who did not qualify for the debate, including Jill Stein and Robert F. Kennedy Jr., have announced their alternative plans for Thursday evening.