With millions of Americans tuning in and the media dissecting their every move, every word, Kamala Harris and Donald Trump will meet for their first public presidential debate on Tuesday.

The Democratic vice president and Republican former president will face off in Philadelphia in their first – and possibly only – televised debate before what promises to be a nail-bitingly close 2024 election.

The high-stakes ABC debate will be a chance for U.S. voters to finally see the two go head-to-head, after a month of shadow-boxing since President Joe Biden threw in the towel as the candidate.

The gloves will be off in what is a critical test for both.

Harris, 59, has turbocharged and unified the Democratic party and will now face an opponent who has called her "crazy" and subjected her to racist and sexist taunts.

America's first female, Black and South Asian vice president has overhauled Trump's lead in the polls but insists she remains the "underdog" in a tight race.

Knowing what's at stake, she is spending five days holed up in the nearby city of Pittsburgh preparing for the debate.

Meanwhile, 78-year-old Trump is expected to opt for an aggressive approach after Harris's entry into the race upended his White House bid and turned him into the oldest candidate in U.S. history.

"These are two very different candidates that have previously never met in person," Erin Christie, of the Rutgers University School of Communication and Information, told the Agence France-Presse (AFP).

"So it will prove to be a very enlightening debate which could even be the make-it-or-break-it factor in the election."

That lack of any prior face time is a result of Trump having refused to attend Biden's inauguration after falsely claiming he was cheated in the 2020 election.

Adding an extra frisson is the fact that the debate is happening in Pennsylvania, the most bitterly fought of the battleground states that will decide the election.

Tuesday's debate could meanwhile be the last. Harris and Trump have not agreed to anymore and this one is only happening after a bitter row ended with Harris's camp reluctantly agreeing to have the candidates' microphones muted while the other is speaking.

Americans will now be watching closely to see how it actually plays out on stage.

'Break out the popcorn'

While opinions differ about how much U.S. presidential debates generally move the polls, there is no doubt they can cause political earthquakes on occasion.

It is after all just over two months since Biden was forced to drop his bid for a second term after a disastrous debate against Trump sparked Democratic concerns about his age and mental fitness.

Biden himself will be watching on Tuesday, his spokeswoman Karine Jean-Pierre said Friday. "The vice president is smart. She is someone that knows how to get the job done," added Jean-Pierre, a former senior aide to Harris during her failed 2020 campaign.

While few are predicting anything quite as dramatic from Tuesday's encounter between Trump and Harris, it still has the potential to be a decisive moment in the final sprint to Nov. 5.

And despite their differences, both will have the same goal – to reach out to a core of undecided voters in a deeply polarized America.

In the red corner, Harris will rely on her coolly cutting style and her history as a prosecutor, as she takes on a convicted felon who also faces charges of conspiring to overturn his 2020 election loss against Biden.

She will still however have to battle sexist and racist stereotypes about "angry Black women," said Rebecca Gill, a political science professor at the University of Nevada, Las Vegas.

While Harris will also face pressure to be less vague on policy, her campaign is expected to keep up the "do no harm" strategy that has seen Harris give just one televised interview since replacing Biden.

In the blue corner, Trump's challenge will be to decide just how much Trump voters want.

Trump's angry, rambling style fires up his right-wing base but it remains to be seen how it will play against a candidate vying to be America's first Black woman president.

All eyes will be on ABC's moderators too to see if they fact-check what will be a stream of falsehoods, if Trump's six previous presidential debates are anything to go by.

"This debate may go down in the history books. Break out the popcorn," said Andrew Koneschusky, a former press secretary for U.S. Senate leader Chuck Schumer.