What to watch for in the Trump-Harris debate

The debate, hosted by ABC News, is set to take place at 9 p.m. ET at the National Constitution Center in Philadelphia. Here are five things to watch for during the first—and only—scheduled debate between the two candidates.

Can Trump stay on topic?

Before President Biden exited the race, Trump’s advisers usually were content to “let Trump be Trump." But with Harris now the Democratic contender, they have ramped up efforts to keep Trump disciplined and on message in the hope that he doesn’t risk alienating some of the critical undecided voters needed to push him over the top in November. His campaign team has urged him to stick to policy in the debate and avoid personal attacks that could backfire with an opponent who is a woman of color.

Trump’s debate advisers, Florida Rep. Matt Gaetz and former Hawaii Rep. Tulsi Gabbard, told reporters on Monday that Trump is particularly focused on the Biden-Harris administration’s economic record and border policies, and will highlight what they call some of Harris’s inconsistencies on issues, including fracking. They said he didn’t need to launch personal attacks when the policy debate speaks for itself.

Nikki Haley, Trump’s former GOP primary foe, said Monday that Trump and his running mate, JD Vance, need to change the way they speak about women.

“You don’t need to call Kamala dumb. She didn’t get this far, you know, just by accident," Haley told Fox & Friends. “She’s a prosecutor. You don’t need to go talk about intelligence, or looks, or anything else. Just focus on the policies. When you call even a Democrat woman dumb, Republican women get their backs up too."

Just as Trump pointed out moments during the first debate when Biden lacked coherence, Harris, 59, has an opportunity Tuesday to criticize Trump, 78, when he says anything that is hard to follow or strays from the question.

Can Harris separate herself from Biden?

Harris has been trying to position herself as the change candidate, as Trump has tried to tie her to Biden’s record, often blaming her for what he views as the administration’s shortcomings. Though she is the sitting vice president, Harris recently has sought, in rhetoric and in substance, to distinguish herself from Biden, who remains unpopular.

As her campaign puts forward policy proposals, Harris has described her vision with the slogan: “A new way forward." And in a recent CNN interview, she contrasted her agenda with her view of the past decade.

“I’m talking about an era that started about a decade ago where there is some suggestion, warped I believe it to be, that the measure of the strength of a leader is based on who you beat down instead of where I believe most Americans are, which is to believe that the true measure of the strength of a leader is based on who you lift up," she said in the interview.

She also has to put some distance between herself and the current administration on policy specifics. She took one step in that direction last week, proposing a less drastic increase in the top capital-gains tax rate than what Biden had outlined in his budget blueprint earlier this year.

Will either of them fill in the blanks on their policy plans?

Both Trump and Harris have talked about their broad agenda for the country, with few specifics on policy proposals.

Harris’s advisers have decided to outline a handful of proposals focused on lowering costs for the middle class instead of issuing lengthy white papers that lay out a comprehensive vision for a second term. Her campaign thinks it is more important to connect with voters on the vision rather than to offer nitty-gritty details that can provide more fodder for attacks. That makes it unlikely she will get in the weeds during the debate—even though polls show some voters say they still don’t know enough about her positions to have a firm opinion.

Trump’s team has said he would focus on what he views as failed economic, border-security and national-security policies by the Biden-Harris administration if he is elected again to office, though he has at times laid out big policy goals without explaining how he would achieve them. His campaign said that Trump will also look to demonstrate that Harris is a “radical Marxist" who poses a threat to economic prosperity in America.

Will the muted microphones matter?

This debate comes after a long back-and-forth over whether the debate should take place, what network should host it, and whether the Harris campaign should be bound to the debate rules agreed to by Biden when he was still the nominee.

This debate will have the same rules as the first presidential debate—meaning a candidate’s microphone will be muted when it isn’t his or her turn to speak. The Harris campaign pushed hard to undo that rule, while the Trump campaign, which once harrumphed over the rule, advocated to keep it.

According to those familiar with the Harris campaign’s thinking, they believed that keeping the mics on opened the door for Trump to potentially say something that could damage his prospects. They also thought she might have had an opportunity to shine more as a prosecutor if she had a combative adversary.

Though Trump himself said at one point that he didn’t care whether the microphones were muted, his campaign thought the muted-mics rule worked well for him against Biden and would help keep his answers more structured.

Which issues will each candidate try to steer the debate toward?

Harris has been focusing her speeches and ads on her economic agenda, tying it to her middle-class roots. She has emphasized the threats to abortion access that have followed the Supreme Court’s overturning of Roe v. Wade, blaming Trump for moves by some states to restrict the procedure. The vice president is expected to stress those issues in the debate and to highlight Project 2025, a conservative policy agenda written by former Trump aides from which the GOP nominee has distanced himself.

Trump, meanwhile, is seeking to blame Harris for the border crisis that many Americans rank among their top election issues. He has hammered Harris for the Biden-era policies that led to a pause in construction of the border wall started while he was president, as well as for a record number of migrants (those numbers have dropped significantly this year) whom he ties to crime, drugs and other issues putting Americans at risk. He has also spoken extensively about soaring inflation and what he calls “Kamalanomics," and he claims that she will hurt the average American household with “communist" economic policies.

Write to Vivian Salama at vivian.salama@wsj.com and Tarini Parti at tarini.parti@wsj.com