Philadelphia, Pennsylvania - Kamala Harris and Donald Trump are poised to face off in a high-stakes televised debate Tuesday, a potentially game-changing moment less than two months ahead of the presidential election.

The stakes could hardly be higher for the Democratic vice president and Republican former president, with tens of millions of American voters expected to tune in at 9:00 PM ET to watch them take the stage for their first and possibly only debate.

A single zinger or gaffe could tip the balance of one of the most dramatic White House races in US history, with the two rivals neck-and-neck in the polls ahead of the November 5 vote.

For Harris (59), it will be a critical chance to win over voters who still know little about her, as her honeymoon starts to fade after suddenly replacing President Joe Biden in July.

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Trump (78) will meanwhile try to box in Harris on issues like the economy and immigration, but may also unleash more of the racist and sexist insults that he's directed her way during the campaign.

Harris, who enjoys a significant advantage among women according to the polls, is expected to press Trump on reproductive rights after he made a number of seemingly contradictory comments recently on abortion access.

The two candidates will be meeting in person for the first time at the ABC News debate in Philadelphia, adding to the potential for a bruising confrontation.

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

The ABC News debate is scheduled to last for 90 minutes and will be held without an audience.

Harris, America's first female, Black, and South Asian vice president, arrived in Philadelphia on the eve of the debate after five days holed up in a hotel doing intense practice sessions.

One of her aides even reportedly dressed up in a Trump-style boxy suit and long tie so she could get used to unloading her best lines on her opponent.

Trump's team said he has taken a more relaxed approach ahead of his seventh presidential debate, choosing to arrive in Philadelphia just hours before and keeping his preparations limited.

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The debate may lack the full-scale shouting matches of previous years, as the two candidates' microphones will be muted when they are not speaking, at the Trump team's request.

But it will still be a potential turning point – as well as a contrast in styles.

In one corner is a former prosecutor who has in the past delivered ice-cold put-downs to debate rivals, including Biden himself and Trump's former vice president, Mike Pence.

"There's no floor for him in terms of how low he will go," Harris said in a radio interview broadcast Monday. "He is probably going to speak a lot of untruths."

In the other corner is Trump, the most brutal knife-fighter in US politics, who has been convicted of falsifying business records to cover up a porn star scandal and is accused of trying to overturn the 2020 election.

"You can't prepare for President Trump," his spokesman Jason Miller said. "Imagine like a boxer trying to prepare for Floyd Mayweather or Muhammad Ali."

Harris, in many ways, has the most to prove.

Her campaign suffered a setback over the weekend when a major New York Times/Siena poll showed Trump ahead by 48% to 47%, and the candidates effectively tied in half a dozen battleground states.

She will be under pressure to spell out her so-far vague policy manifesto to voters, who, according to the NYT poll, said they need to know more about her.

Former reality TV star Trump is by far the more experienced presidential debater, with six under his belt, but swing voters may be put off if he insults the candidate aiming to become America's first woman president.

Trump is still reveling in the fact that his last debate opponent, 81-year-old Biden, performed so catastrophically that he was forced to withdraw from the race.

The vice presidential candidates – Democrat Tim Walz and Republican JD Vance – are to hold a debate on October 1.