Philadelphia: Kamala Harris and Donald Trump met Tuesday for their first -- and possibly only -- televised debate, a potential game-changer in a presidential race that has already cast the United States into historic upheaval. The Democratic vice president and Republican former president entered the venue in the National Constitution Center in Philadelphia. Then they shook hands -- to the surprise of many -- before heading to their podiums. The live clash on ABC News, watched by tens of millions of voters, was expected to last around 90 minutes.

With only 56 days left before the November 5 election, the intense spotlight will be a rare opportunity for both candidates to shift the balance in what polls show is an almost evenly split contest.

Much of the attention was on whether Trump will fire his characteristic insults at Harris -- or whether the former California prosecutor will get under the skin of convicted felon Trump.

A single zinger or gaffe could dominate TV screens and social media platforms for days to come. And the debate is a key chance for Harris to introduce herself to more voters after only jumping into the race less than eight weeks ago, when 81-year-old President Joe Biden abruptly quit.

Trump, 78, touched down in his plane -- dubbed Trump Force One -- just over two hours ahead of the debate, while Harris, 59, arrived in the eastern city on Monday.

The pressure is arguably greater for Harris, America's first female, Black and South Asian vice president, as she takes part in her first presidential debate.

Former reality TV star Trump will attack Harris on issues like the economy and immigration, but may also unleash more of the racist and sexist invective that he's directed her way during the campaign. Both candidates are vying to be seen as championing change.

Harris's favorite rally slogan is "we're not going back" to the chaos of the Trump years, while Trump is working to define Harris as responsible for much of the unpopular policies in the Biden administration.

Under the rules, the debate was being held without an audience, while the rivals' microphones were only to be on when it was their turn to speak in order to avoid interruptions.

'Calm, cool and collected' 
The last presidential debate in June doomed Biden's reelection campaign, after he delivered a catastrophic performance against Trump. Harris took over as nominee amid Democratic fears that Biden was too old and infirm to defeat the scandal-plagued Republican.

Biden told reporters he would watch the debate from New York -- where he traveled on Tuesday ahead of ceremonies marking the anniversary of the September 11, 2001 attacks.

"I spoke to the vice president. She seems calm, cool and collected. I think she's going to do great," Biden said.

Tim Smith, 39, a Harris supporter, stood outside the debate venue holding a sign similar to those used by the Trump campaign but bearing the word "Loser." Smith said he was hoping "we'll get to hear a little bit of the policies they want to institute -- and not so much the name calling or the attacks.

Inside the venue, both candidates' teams were working the "spin room" hours ahead of the debate.

The Democrats brought along two former Trump administration officials who are now harsh critics, including the short-lived White House communications director Anthony Scaramucci.

California Governor Gavin Newsom, a prominent Harris supporter, called her "the next generation of leadership in this country."

"It's daylight, and darkness, chaos versus competency, right and wrong, liberalism and illiberalism," he said.

Trump the 'boxer'
Harris has earned a reputation in past debates and while serving as a senator for ice-cold put-downs and tough questions. But Trump is the most brutal public speaker in US politics. He also benefits from an incredible ability to survive any scandal -- and his fervent base if anything enjoys his frequent gaffes, fabricated stories and promotion of conspiracy theories.

He has been convicted of falsifying business records to cover up an affair with an adult film star, found liable for sexual abuse, and faces trial on charges of trying to overturn the 2020 election that he lost to Biden. But he may surprise by opting for a more restrained performance, as he did in June, allowing Biden to self-destruct.

"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."