It will be the first time Kamala Harris and Donald Trump meet in person -- and millions of Americans will get a ringside seat.
The Democratic vice president and Republican former president will face off in Philadelphia on Tuesday 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 US 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 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.
The 78-year-old Trump is meanwhile 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 US 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 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 any more, 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 US 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 on 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 November 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.
But 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 US Senate leader Chuck Schumer.
Trump v. Harris debate: what are the rules?
US Vice President Kamala Harris and former president Donald Trump will face off in a debate for the first time on Tuesday, each seeking an edge in what has become a neck-and-neck race for the White House.
After weeks of back-and-forth on the where, when and how of the debate, US broadcaster ABC -- which is hosting the event -- announced the final rules of the contest, as agreed to by the two campaigns, this week.
Here are the major takeaways.
- When and where? -
The debate will take place at the National Constitution Center in Philadelphia at 9:00 pm local time Tuesday (0100 GMT Wednesday,) and will be aired live. There will be no audience present in the room.
The event will be moderated by ABC anchors David Muir and Linsey Davis, and will run for 90 minutes of debate time, with two commercial breaks.
- Muted mics? -
The issue of whether microphones would be muted -- as they were during a previous debate between US President Joe Biden and Trump -- was a contentious one, with each side accusing the other of cowardice over the question.
ABC has announced that each candidate's microphone will only be live for when it is their turn to speak, and muted when the time belongs to the other candidate.
Only moderators will be allowed to ask questions, and no topics or questions will be shared in advance with the candidates.
- Who speaks when? -
Candidates will not offer opening statements. They will be allotted two minutes to answer each question, with two minutes granted to their opponent for a rebuttal.
There will be an additional minute for "a follow-up, clarification, or response," according to the rules.
At the end of the debate, each candidate will offer a two-minute closing statement, with Trump to go last, as per the results of a virtual coin toss.
- No props -
Candidates will stand behind podiums through the entire length of the debate, with no props or pre-written notes allowed.
Trump and Harris will each be given a pen, a pad of paper, and a bottle of water.
Campaign staff will not be allowed to speak or interact with the candidates during the commercial breaks.