Debate day is here: 5 things to watch for in the high-stakes Harris-Trump faceoff

Vice President Kamala Harris and former President Donald Trump square off Tuesday at what could be their only presidential debate, setting high stakes for an event expected to be viewed by millions of Americans and a key sliver of undecided voters.

Harris' momentum -- after her unusual rise as the Democrats' nominee shot her into a neck-and-neck race -- has now stalled, making the head-to-head matchup an opportunity to get that started again -- If she can adequately make the argument for her own candidacy and cast Trump as unfit for another term.

Trump, meanwhile, has struggled to find a way to consistently and effectively attack his new opponent but has remained highly competitive, thanks to a large base of immovable supporters and the broad swath of Americans who already have fixed views of him. Tuesday's debate offers him an opportunity to solidify his support while painting Harris in a negative light to an electorate that has less cemented perceptions of her.

The ABC News debate, moderated by David Muir and Linsey Davis, will take place on Tuesday, Sept. 10 at 9 p.m ET. A prime-time pre-debate special will air at 8 p.m. ET. It will air on ABC and stream on ABC News Live, Disney+ and Hulu. Viewers can also stream the debate on the ABC app on a smartphone or tablet, on ABC.com and connected devices.

Here are five things to watch at Tuesday's debate.

Can Trump stay focused on policy?

Trump has worked to peg Harris as a "California liberal," letting voters' perceptions of the progressive bastion paint a picture of a candidate who served as the state's attorney general and junior senator. He's also hammered her on immigration and inflation -- two voter concerns on which polls suggest he has an edge.

However, he's also veered into personal remarks, including falsely questioning Harris' racial identity (she's Black and south Asian), touting what he says are his superior good looks and promoting vulgar and false allegations that past romantic relationships of Harris' helped propel her political career.

Staying on message on his four-year economic record, which saw low inflation before the pandemic and less fervor over unauthorized border crossings, is key, allies told ABC News.

Veering into personal attacks would be counterproductive, they argued, drawing media attention away from what they view as a favorable policy issue set.

"I think he does," former White House press secretary Sean Spicer said when asked if Trump makes an effective contrast on the trail. "If I had a critique, it would be that he'll make the case sometimes and then, with all due respect, he will sometimes go beyond the case and give the media something else to focus on."

"He needs to avoid creating a moment that takes the focus away from her record. So, if it's about her personality or her appearance as opposed to her record, that will change the focus of what people talk about the next day."

How does Harris introduce herself to undecided voters?

While Trump comes in with nearly universal name recognition, 28% of likely voters in a recent New York Times/Siena College poll said they feel they "need to learn more about Kamala Harris." That means that while many voters have heard of Harris, she is less defined than Trump in their eyes and thus has more work to do to introduce herself -- lest she be defined by her opposition.

Harris has dual goals in Tuesday's debate: make the case for herself as someone who would be a capable president and get under Trump's skin to spark a reaction to suggest he isn't worthy of another four years in the White House.

"I think there needs to be a long litany of just pummeling Donald Trump while also being extremely clear about what your vision is for the future," said Bakari Sellers, a prominent Harris ally and Democratic media commentator.

Harris on the trail has sought to do both.

Monday, she fleshed out her policy proposals in a new page on her website, her most expansive explanation yet of her platform. And in early stump speeches, she boasted of her time working as a prosecutor and state attorney general combating gangs and other criminal activity, saying to crowds that she knows "Donald Trump's type," in a clear reference to his legal travails.

The way she balances those two dictates could offer clues as to the way she and her campaign best think she can march to victory in November.

Will there be any hot mics?

Harris had a memorable debate performance in 2020, when she faced off against then-Vice President Mike Pence. Pence was muscling in on her answers, allowing her to declare, "I'm speaking," in one of the more viral instances of the night.

It's unclear whether she'll be able to replicate such a moment.

The candidates' microphones will be muted while their opponent is answering a question, something Harris' team argued against in the hopes of tempting Trump to aggressively interrupt her and come off as unpresidential.

In an election in which policy is largely taking a backseat to personality, producing such a clash might possibly spark one of the debate's most notable moments.

How big of a role will President Joe Biden play?

Harris has been walking a tightrope since the start of her campaign between recognizing her role in Biden's administration and touting its achievements while also casting herself as a candidate in her own right, particularly after the unusual way in which she became her party's nominee.

A recent New York Times/Siena College poll showed that roughly 61% of likely voters said the next president should represent a major change from Biden. Only 25% of them said Harris represented that change, compared to 53% who said Trump did.

Harris so far has appeared mostly on the campaign trail by herself, and in a joint appearance in Pittsburgh and at Democrats' convention last month, Biden spoke first before handing the stage off to Harris, underscoring her role in the electoral spotlight.

Trump, meanwhile, has at times focused extensively on Biden, particularly in the days and weeks after the president ended his campaign and handed the reins to Harris.

Such a strategy risks focusing too much on Biden rather than Trump's own opponent -- but, allies said, tying Harris to voter disapproval of the way the current president has handled the economy and inflation could be a boon.

"For him to be viewed as having a successful debate, he has to continue that assault," said one former campaign aide in touch with Trump's current team. "She's the vice president United States seeking the second term of Joe Biden. We can make that case."

Is there a major moment that moves the electoral needle?

The last debate between Biden and Trump was clearly consequential -- it ended the former's campaign. That doesn't mean Tuesday's debate will pack the same punch.

Surely, millions will tune in to the latest salvo in a race packed with unpredictable twists and turns, raising the stakes. But many debates make little more than ripples in presidential races -- an outcome that might benefit neither candidate.

As it stands, it's a neck-and-neck race. Harris would like a moment that revives her momentum, which jolted her into contention but now is stalled; Trump would like a moment to erase some of the gains Harris has made and actually reverse her improved poll numbers.

Now it's up to the candidates to make it happen.