Mr. Trump had his staff fire up his campaign playlist, standing on the stage for about half an hour and swaying to songs as his crowd slowly dwindled.

He bobbed his head through the Village People’s “Y.M.C.A.,” his usual closing song. He swayed soberly to Rufus Wainwright’s version of “Hallelujah,” watched a Sinead O’Connor video, rocked along to Elvis, watched the crowd during “Rich Men North of Richmond” and then, finally, left the stage to shake hands on his way out during one last song.

The impromptu D.J. session was a strange conclusion to a political event that had started on familiar turf. Aided by Gov. Kristi Noem of South Dakota, Mr. Trump answered questions in front of hundreds of people at the Greater Philadelphia Expo Center in Oaks, Pa., about 18 miles northwest of Philadelphia.

The inquiries from friendly audience members allowed Mr. Trump to rattle off a series of talking points about the economy and immigration and attacks against Vice President Kamala Harris. But the event’s tone shifted about 30 minutes in, when two people in the crowd needed medical attention after apparently passing out.

As medical personnel responded to the first incident and the room grew tense, Mr. Trump asked his campaign staff to play a rendition of “Ave Maria.” Minutes later, after the person was removed on a stretcher, Ms. Noem tried to return to the question-and-answer session. But that effort stopped abruptly because a second person, a woman, also suddenly needed medical attention.

After the woman got up and walked to the periphery of the event, Mr. Trump called for some air conditioning in the venue. Ms. Noem, trying to get back on message, made a joke about inflation, saying, “They probably can’t afford it, sir.”

Then, Mr. Trump suggested a return to his comfort zone. He called up a chart on immigration that he has displayed at nearly every recent campaign rally and ordered up “Ave Maria” again. And after it played one more time, Mr. Trump decided to end the question-and-answer session after just five questions.

“Let’s not do any more questions. Let’s just listen to music,” he said. “Let’s make it into a musical. Who the hell wants to hear questions, right?”

Mr. Trump’s crowd cheered in approval, an indication of how his supporters flock to his rallies to be in his presence as much as to hear him make political points with which they are intimately familiar.

The playlist session was a glimpse of the private version of Mr. Trump seen more often at Mar-a-Lago, his residence and club in Palm Beach, Fla., than at political events. The former president has been known to take out an iPad that is connected to the speaker system there and play D.J. for his guests.

As Mr. Trump stood through the first few songs, basking in the admiration of his supporters, the crowd packed into the expo center largely stayed put, filming him on their phones and at points singing along.

Mr. Trump’s supporters often wait for hours to see his rallies, and there can be lengthy lines for food, water and bathrooms once they are inside. At several outdoor events this summer, attendees have needed medical attention because of heat-related illnesses.

But Mr. Trump generally returns to his planned remarks after medical issues at other events. On Monday, he seemed more uncertain how to proceed. After offering what appeared to be a closing statement and having his campaign play a James Brown song, Mr. Trump suggested taking another question or two. As the crowd cheered in approval, he said, “let’s go,” but then said he’d play “Y.M.C.A.” and send the crowd home.

But after “Y.M.C.A.” ended, Mr. Trump seemed a little perplexed. “There’s nobody leaving,” he said. “What’s going on?” The audience cheered, and so the music kept going, as Ms. Noem stood awkwardly by, and many in the audience seemed unsure about whether the event was over.

Still, as one song became two, then three, many of those in the back of the house began to filter out. As the opening chords of the Guns N’ Roses power ballad “November Rain” played, one of Mr. Trump’s aides, Justin Caporale, came on the stage with a sheet of paper.

Mr. Trump briefly reviewed what had been handed to him, and the two chatted briefly. Then, seconds later, Mr. Trump decided he’d had his fill. He waved, pumped his fist, and finally made his way off the stage.

“After all these years, we know who Donald Trump is,” Ms. Harris said. “He is someone who will stop at nothing to claim power for himself.”

In a striking moment, Ms. Harris told the crowd of 6,000 that they didn’t have to take her word for it, that she had an example of his “worldview and intentions.”

“Please — roll the clip,” she said as the crowd groaned and gasped as Mr. Trump’s face flashed on screens.

“He’s talking about the enemy within our country, Pennsylvania,” Ms. Harris said to a jeering crowd. “He’s talking about that he considers anyone who doesn’t support him, or who will not bend to his will, an enemy of our country.”

Ms. Harris’s visit to Erie County, a bellwether county that is crucial to a statewide victory in a state that could be the 2024 tipping point, was her first as the Democrats’ presidential candidate since President Biden ended his re-election bid and backed her. Mr. Trump won the county in 2016 but lost it narrowly to Mr. Biden in 2020.

Ms. Harris used the first half of her stump speech to lay out her policy proposals to build an “opportunity economy,” taking on price-gouging and expanding Medicare to cover home health care for the elderly and ill. She continued positioning herself as the underdog and argued that the stakes of this election were higher than in 2016 and 2020 because of a Supreme Court decision that vastly expanded presidential power.

She called Erie a “pivot county” and pleaded with those in the crowd to cast their ballots before November, as early voting has already begun in the state. “The election is here,” she said.

But it was the back half of her speech where Ms. Harris ramped up her attacks on Mr. Trump. She has increasingly engaged in the unusual campaign strategy of drawing attention to her opponent’s rallies. It was at their debate last month where she first encouraged people to watch his rallies as she called attention to violent rhetoric and erratic behavior. On Monday, she issued her most ominous warning yet, outlining how he has attacked officials who don’t find extra votes for him in an election, judges whose rulings he disagrees with and journalists whose coverage he doesn’t like.

“This is among the reasons I believe so strongly that a second Trump term would be a huge risk for America, and dangerous,” she added. “Donald Trump is increasingly unstable and unhinged.”

Ms. Harris’s campaign on Monday also released a new ad called “Enemy Within,” drawing on Mr. Trump’s recent comments, featuring two of his former national security aides, Olivia Troye and Kevin Carroll.

“I do remember the day that he suggested that we shoot people on the streets,” Ms. Troye recalls in the ad.

“A second term would be worse,” Mr. Carroll says.

The advertisement is similar to one the campaign began in battleground states this month, with top national security officials issuing dire warnings about Mr. Trump’s fitness for office.

The Democratic ticket’s attacks were twofold on Monday night. In Green Bay, Wis., Ms. Harris’s running mate, Gov. Tim Walz of Minnesota, struck perhaps his most fiery tone yet against Mr. Trump. Mr. Walz pointed to his own military service as he denounced Mr. Trump’s remarks, saying the notion of using the military against American citizens made him “sick to his stomach.”

Mr. Walz urged the audience to realize Mr. Trump’s proposals were not normal.

“The former chairman of the Joint Chiefs of Staff, Mark Milley, said, ‘No one has ever been more dangerous to this country than Donald Trump, and he is a fascist to his core,’” Mr. Walz said, referring to statements from the former officials featured in the advertisement.

“Let that sink in, and don’t be a damn bit afraid of saying it, because that’s exactly who he is.”

Mr. Trump visited Erie County last month, where he railed against Ms. Harris, telling the crowd that she should be “impeached and prosecuted” for her handling of the southern border and suggested that “one really violent day” would quell crime in American cities.

As Election Day draws near, Ms. Harris has been goading Mr. Trump about his mental fitness, attacks that have intensified amid his rambling speeches at rallies.

Ms. Harris, who has insisted that Mr. Trump agree to a second debate, has maintained that he is hiding from engaging in normal campaign activity, like a “60 Minutes” interview. Over the weekend, she released her medical records, which found her in “excellent health,” and used another rally to question why Mr. Trump has not released his.

“It makes you wonder, why does his staff want him to hide away?" she said during a rally in North Carolina on Sunday. “One must question, are they afraid that people will see that he is too weak and unstable to lead America? Is that what’s going on?”

But during the rally, Ms. Harris also drew one line when it came to her opponent. After quoting him as saying that he wanted to “terminate the Constitution,” the crowd began chanting, “Lock him up.”

“Here’s the thing: The courts will handle that,” she said. “Let’s handle November, shall we?”

Jazmine Ulloa contributed reporting from Green Bay, Wis.

“We’re very concerned about Trump dividing the country, civil rights, everything that’s happening,” Mr. Siblani said. But Ms. Harris’s association with the Biden administration’s backing of Israel in the regional war, he said, also made it impossible to endorse her. “She has not separated her ideas from Biden’s,” he said.

Arab Americans have broadly voted Democratic for more than two decades, and neither AAPAC nor the Arab American News has endorsed a Republican for president since George W. Bush, though both have occasionally declined to endorse candidates.

Mr. Siblani has long been frank in his opposition to Israel, which he has called a “terrorist state,” and recently hailed Hassan Nasrallah, the Hezbollah leader assassinated by Israel, as “the great leader of this time.” But his publication has nonetheless endorsed candidates from both parties who affirm the U.S. government’s longstanding support for Israel, and emissaries of both presidential candidates have met with him this year.

Polls shortly before and after the 2020 election suggested that Mr. Biden enjoyed the support of both Arab American and Muslim voters by a large margin in that election. But Arab Americans have turned sharply against his administration over its backing of Israel after the Oct. 7 Hamas attacks, which Israeli officials say killed about 1,200 people with more than 200 others taken hostage. Israel’s retaliatory military operation has killed 41,000 in the Gaza Strip and 2,000 in Lebanon, according to government health ministries.

The war has become a potential political liability for Ms. Harris, who has sought to walk a fine line on the conflict to avoid alienating constituencies on either side. Her path to victory in November is likely to rely on holding several northern swing states won by Mr. Biden in 2020, including Michigan — where Arab Americans constitute a larger share of the overall population than any other state. (AAPAC also declined to endorse a candidate in the state’s Senate race between Representative Elissa Slotkin, a Democrat, and former Representative Mike Rogers, a Republican.)

Some groups that have been critical of the Biden administration’s Middle East policy have endorsed Ms. Harris, whether enthusiastically or ambivalently, or at least hinted that she is the better option.

In September, Ms. Harris was endorsed by Emgage Action, one of the country’s largest Muslim voter-mobilization organizations, though that group has continued to call on her to support a cease-fire in Gaza.

And the Uncommitted movement, a group of activists within the Democratic Party that had sought to push Mr. Biden toward endorsing a cease-fire and arms embargo by voting against him in the primaries, has recently grown more vocal in its opposition to Mr. Trump. In a video, a founder of the group said bluntly that a Trump victory would be worse than a Harris presidency — but stopped short of endorsing her.

This is Ms. Harris’s first formal interview with Fox News, whose day-to-day programming is heavy on conservative punditry that often explicitly supports her Republican opponent, former President Donald J. Trump.

It could also represent an opportunity for the Democratic nominee three weeks ahead of Election Day.

Ms. Harris will have a chance to deliver her message to a viewership that may be skeptical of her candidacy. Her willingness to appear on Fox News may aid the perception that she is open to facing tough questions. And she can reach a swath of independent voters, more of whom watch Fox News than CNN or MSNBC, according to research by Nielsen.

Senior Democratic officials have long shown hostility toward Fox News, going so far as to formally bar the network from hosting a primary debate in 2020. Hillary Clinton, in 2016, was the last Democratic presidential nominee to sit for a Fox News interview. President Biden has not appeared on the network since taking office, though he has jousted at news conferences with its senior White House correspondent, Peter Doocy.

But a thaw has occurred.

Ms. Harris’s running mate, Gov. Tim Walz of Minnesota, was interviewed on “Fox News Sunday” the past two weekends. (Mr. Walz’s aides reached out to Fox to schedule his second appearance.) In recent months, the network has also welcomed a string of Harris supporters, including Gov. Gavin Newsom of California, Gov. Josh Shapiro of Pennsylvania, and Transportation Secretary Pete Buttigieg, who is now such a regular that he cheekily told Democratic convention-goers, “You might recognize me from Fox News.”

Mr. Baier’s interview with Ms. Harris is scheduled to air on the same day that Fox is set to broadcast an unusual town hall in which Mr. Trump plans to field questions on subjects like abortion, child care and day care from an all-female audience.

Ms. Harris has appeared across a range of traditional and niche media outlets in recent weeks.

On Monday, she used interviews with Roland Martin and The Shade Room, an online entertainment publication, to further her pitch to Black voters, arguing that Mr. Trump had engaged in a decades-long pattern of racist behavior.

On Tuesday, she is set to record a live interview in Detroit with Charlamagne Tha God, host of the syndicated morning radio show “The Breakfast Club,” which is particularly popular with Black millennials.

Last week, she sat for interviews with “The View,” “The Late Show with Stephen Colbert” and “The Howard Stern Show.” Ms. Harris sat for a “60 Minutes” interview that aired on CBS last week. Mr. Trump refused to appear on the program and accused CBS of bias; the network said he had committed to an interview and then reneged.

Mr. Trump is a frequent presence on partisan Fox News shows like “Hannity”; he has also kept up a heavy schedule of interviews on podcasts and other alternate media, including a video game celebrity’s streaming page. On Tuesday, he will be interviewed by the editor in chief of Bloomberg News, John Micklethwait, at the Economic Club of Chicago.