WILKES-BARRE, Pa.—Anyone who doubts that the road to the White House runs through this blue-collar city can just ask Donald Trump . He has been here repeatedly as a candidate since 2016, holding raucous rallies at the spacious Mohegan Sun Arena.

Or ask the fans who started lining up in the early morning hours Saturday for his fifth rally at the arena. Trump’s repeated visits are a clear sign he still believes the key to winning Pennsylvania, the nation’s largest battleground state, is to boost support among voters unhappy over how the Democratic Party has led the country over the past four years. And in the process, he hopes to turn formerly Democratic counties, such as this one, into GOP strongholds.

“I got so fed up with how the Democratic Party has gotten so liberal,” said Stephen Kotch, a registered Democrat and retired employee for the state’s liquor control board, who had just left the Luzerne County Republican Party’s headquarters. He said that he has voted for Trump in both prior elections.

Republicans know that they have an opportunity—and Democrats know they face a challenge—in countering Trump’s big gains in rural and blue-collar parts of the state. Vice President Kamala Harris and her running mate, Minnesota Gov. Tim Walz , will barnstorm Western Pennsylvania Sunday, and then Trump will be back in the state Monday to give remarks on the economy in York.

Joe Biden won Philadelphia County by more than 470,000 votes in 2020, but he won the overall state by only about 81,000 votes. For Trump, driving up the numbers in places like Wilkes-Barre is the key to repeating his 2016 win in Pennsylvania.

Wilkes-Barre is a logical place for Trump to try to peel off Democrats, said Neil Makhija, a Democratic county commissioner in suburban Montgomery County, outside Philadelphia.  Makhija said the Wilkes-Barre area has made one of the state’s biggest shifts toward the GOP, in large part because voters who were once union Democrats have struggled as steel, coal and manufacturing jobs disappeared and in some cases moved overseas. “It’s been the prime target for Trump’s message when it comes to tariffs, when it comes to immigration, messages against globalization,” he said.

Shawn Winders, 48, a registered Republican who had worked at a coal-fired power plant before it was retired, is one example.

“I lost my job,” he said, noting that coal had been a major part of the state’s natural-resources economy. Pennsylvania is also rich in natural gas, ranking as the second-biggest producer in the U.S. thanks to fracking. Harris called for banning the technique in 2019 when she was a presidential contender—an unpopular position here that she no longer holds.

“She’s flip-flopping just to get elected,” said Dawn McIntyre, a 54-year-old from Luzerne County. “She’ll back out on it when she’s in office. And it seems like anything that Donald Trump wants to do now, she’s for it, to try to bring some Trumpers over.”

At his rally, Trump cited Harris’s prior opposition to fracking and called her a socialist. He said that Democrats had rigged their presidential nominating contest by elevating Harris without giving rank-and-file Democratic voters the chance to say who should step in after Biden opted to quit the race . Some voters at the rally agreed.

“Her Democrat people did not vote for it,” said Jana Bangs, a 43-year-old registered Republican from Columbia County, who was tailgating at a mall parking lot hours before the rally began. “I feel like you’re taking away the people’s rights, what we were built on, by just sliding her in.”

Trump also complained about flattering remarks observers have made about Harris’s personal appearance.

“I am much better looking than her,” he told the crowd. (Trump called Harris “a beautiful woman” during an Aug. 12 interview on X with Elon Musk , comparing her looks to those of his wife, Melania Trump.)

Trump didn’t mention abortion, an issue that some voters here said was their top priority. Democrats have used the topic, particularly the Supreme Court’s overruling of Roe v. Wade , to drive up support among women.

“If you can say there’s life on Mars, why can’t you say there’s life in there?” said Tony Blackwell, a truck driver wearing a “Fight, Fight, Fight” T-shirt who lives near Williamsport, Pa., pointing to his stomach. “That’s a big thing for me and my family.”

Responding to the rally, Harris campaign spokesman Joseph Costello said the Democratic nominee was unifying the country while Trump resorted “to lies, name-calling, and confused rants.”

Trump was the first Republican presidential candidate to win Luzerne County, home to Wilkes-Barre, since 1988. And when it shifted, the shift was dramatic: Trump won the county by nearly 20 points in 2016 and then by 14 points in 2020.

Republicans have outsourced some of their registration efforts in battleground states to a group affiliated with the activist Scott Presler called Early Vote Action. It has paid off: So far this year, some 1,355 Democrats in Luzerne County have switched their party affiliation to Republican, according to data from the secretary of state’s office. That compares with 221 Republicans who have switched to become Democrats.

On Saturday, a group of self-described “MAGA Boys” associated with Presler were walking up and down the lines of people streaming in to see Trump, working to register new Republican voters.

“The Democrats are getting nervous,” said Nick Passino, one of the MAGA Boys. “They’re starting to put more of an effort here, and once we see them start getting nervous, we know we’re on to something.”

Write to Siobhan Hughes at Siobhan.hughes@wsj.com and Aaron Zitner at aaron.zitner@wsj.com