Joe Biden and Donald Trump square off 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 polarised 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.

For many voters, the choice between President Biden, the oldest ever incumbent, and Trump, now a convicted felon, is an uninspiring one — a dynamic both candidates need to counter as they go head to head.

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 81-year-old Biden will be desperate to avoid any major gaffes that could underline concerns about his age, and to focus on his key campaign message that Trump poses an existential threat to US democracy.

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 candidates 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.

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 per cent to 45pc.

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, eschewing dress rehearsals in favour 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 characterised 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.

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.