Donald Trump often steals the show, but this debate's spotlight will be on Kamala Harris

For Kamala Harris, Tuesday night's presidential debate with former U.S. president Donald Trump could be a significant moment of opportunity for the vice-president, but also, perhaps, a significant risk. 

"I do think this is a debate about Kamala Harris more than it's a debate about Trump," said Alan Schroeder, professor emeritus at Northeastern University's school of journalism, and an expert on presidential debates.

With Trump such a known commodity, the debate — the first and possibly only contest between the two current candidates — could offer Harris the occasion to grow her level of support. On the flip side, she could alienate voters if she does not perform well.

"Trump has been so dominant on the American political stage for the past decade, really," Schroeder said. "And now it's about someone else."

That could be an advantage for Harris, says Schroeder, because there will be more attention on her. But it also means more pressure and "the absolute need for her to have a successful debate, in order to capitalize on this momentum."

But for Trump, experts say a poor performance — which may include his habits of avoiding answering questions, making false statements and repeating personal attacks he's made against Harris on the campaign trail — could have more resonance this time without the distraction that President Joe Biden's performance provided last time.

"Biden was so awful that it actually overwhelmed the fact that Trump did not have a good debate performance either," said Patrick A. Stewart, a political science professor at the University of Arkansas, and the author of The Audience Decides: Applause-Cheering, Laughter, and Booing during Debates in the Trump Era.

Stewart also believes Trump, who has "never been a policy guy," may also be judged on whether he still has the same vigour he's had in prior debates.

"Is it still the Trump that we know from back in 2016 and 2020 where he was sharp, was on his game as far as verbally being able to come at people and verbally attack them," he said. "Or will he have lost a little bit of steam?"

The debate, which is scheduled for Tuesday at 9 p.m. ET, is expected to last 90 minutes and is being moderated by ABC News anchors David Muir and Linsey Davis, with no studio audience.

It may not have the same immediate impact as Trump's debate against Biden, whose poor performance raised questions about his cognitive abilities, and eventually led to him stepping down as the Democratic presidential candidate before being replaced by Harris. 

Still, with polls suggesting the race is tied, either candidate's debate performance could move the needle among those undecided voters who will be key in determining the outcome of the election. 

"I think this will be sort of a big, marquee moment for people to begin to arrive at a decision," Schroeder said.

For weeks, Harris has reportedly been preparing for this debate, which has included mock debate sessions. She's expected to focus on what her team perceives as Trump's policy failures as president, which include the U.S. border wall, infrastructure and the COVID-19 pandemic, Reuters reported.