Trump vs Harris Presidential Debate: Candidates go on attack in zinger-filled showdown

Harris and 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. Here's what to look out for.

Can Trump stay focused on policy? Staying on message on his four-year economic record is key, while veering into personal attacks would be counterproductive, allies told ABC News.

Harris has dual goals: make the case for herself as someone who would be a capable president and get under Trump's skin. How will she introduce herself to undecided voters?

Read the rest of the five things to watch for in the debate here.

President Joe Biden will be watching the debate from New York, where he's going to be for 9/11 memorial events, according to White House press secretary Karine Jean-Pierre.

"The president will definitely be watching," she said in Monday's briefing.

Jean-Pierre wouldn't give details on any conversations about the debate between Harris and Biden or whether he's given her any takeaways from his own last face-off with Trump, but she said the president was "very proud" of Harris.

"What I can say is, that he'll be watching, he supports, obviously, the vice president, is very proud of her, and I just don't have anything else to add," she said.

Gov. Tim Walz, Harris' running mate, previewed the vice president's upcoming debate with Trump in a fundraising email on Tuesday morning -- contrasting their campaign's message on the economy and freedom with Trump's platforms.

"She's going to show everyone watching what this movement is all about: supporting families like yours and mine and building a future where everyone has the opportunity not just to get by, but to get ahead," the email reads. "Donald Trump is sure to talk about his twisted version of freedom," Walz claimed in the email.

How the Harris/Trump presidential debate is expected to boost Philadelphia's economy

While there is no live audience, the debate is still drawing in hundreds of people, from the media to politicians and of course staff members -- including security for both the Harris and Trump campaigns.

People on both sides of the aisle agree on how the debate will affect the city's bottom line.

"It's normally very good for the local economy," said Marques Reed of Bassetts Ice Cream.

Some added that hosting the debate in Philadelphia is creating a huge opportunity for businesses across the city.

"It will fill our rooms for several days because you have the Secret Service advanced team coming in ahead of time, you have the media coming in. And not only that, when I heard it was early in the week I also knew we'd get a lot of Sunday night stays, which are very coveted in our industry," said Ed Grose, executive director of the Greater Philadelphia Hotel Association.

Without the debate, experts say this would have been a tough time for the hospitality industry, which employs 70,000 people in the city.

"We don't have any conventions in town," Grose said. "We've lost an Eagles home game because they're in Brazil, so this is a very much-needed shot in the arm for us."

Hotels have not fully recovered from the pandemic, experts say, expecting to finish the year at around 64% occupancy versus over 80% pre-pandemic.

"There will be extra hours for people working in the hospitality industry. Also very good for Uber drivers, very good for bars, restaurants," he said.

"I think it's great. It'll bring more people into the city. They'll see how beautiful our city is," said Carmen of Carmen's Famous Italian Hoagies and Cheese Steaks at Reading Terminal Market.

And to top it all off, visitors won't only be getting to experience the debate, but Philadelphia culture as well.

"It's just a good chance to meet some new people and have some new people try some new things," said Reed.

Business owners say they hope those who are visiting get a full feel for Philadelphia and continue coming back.