ALIQUIPPA, Pa. — This downtown 25 miles northwest of Pittsburgh, marked by chains of shuttered and abandoned storefronts, bears all the exit wounds of the manufacturing era.

“When the mill shut down . . . people left here in the middle of the night, walked away from mortgages,’’ said city administrator Samuel L. Gill. “It was a traumatic time. We saw our population drop overnight by 35 to 40 percent.’’

Similarly struggling towns across Pennsylvania are proving to be fertile ground for Republican Donald Trump, who leads in state polling for the April 26 presidential primary.

For many, Trump’s promises of restored American greatness echo President Ronald Reagan’s “city on a hill’’ theme from the 1980s — before the towns’ economic drivers went overseas. In his historic victories, Reagan appealed to what would be known as “Reagan Democrats,’’ mostly blue-collar whites uneasy with national trends.

Now, those voters’ continued disaffection, and signs that Republicans are making gains here, have some Democrats nervous. According to state registration data, Republicans have greatly outpaced Democrats among party-jumpers. In Aliquippa’s Beaver County, nearly seven times as many Democrats have changed their party affilation to Republican as the other way around.

“That’s an interesting county that’s in transition, [they] were diehard Democrats for decades,’’ said Jack Hanna, former southwestern caucus chairman for the state Democratic Party. “The typical western Pennsylvania Democrat from years past is what I would call culturally conservative, and it’s finally starting to catch up with us.’’

Gill agreed that Trump had found some backing even in traditionally Democratic areas, saying, “He appeals to the working class with the possibility of bringing back manufacturing jobs. That’s the key thing they want to hear.’’

With Trump holding the lead in the Republican primary, but doubts still lurking about his ability to close the deal in November, places like Aliquippa are acquiring a growing importance in the campaign. Over the next several weeks, states with heavy rural and manufacturing roots — including New York, Indiana, Nebraska, West Virginia, and Kentucky — will vote in the Republican presidential primary.

Trump’s scalding critique of US trade policies over the last several decades resonates in these former manufacturing hubs. The concept of American decline echoes their own experience, particularly in bustling mill towns that have gone fallow as urban centers thrive. While cities like Pittsburgh, just up the Ohio River, are embracing a new economy with inventively refashioned neighborhoods and a wealth of cultural offerings, smaller towns barely hang on to the vestiges of the good times.

“I will vote for Trump,’’ said Bill Rowan, a 66-year-old Vietnam veteran living in Beaver, just north of the Ohio River from Aliquippa, who said he voted twice for President Obama. “As crazy as it is, as difficult as he is as a politician, he says a lot of things that people want to hear.’’

In Aliquippa, the Jones & Laughlin steel complex, which once employed 17,000 workers and served as the town’s economic engine, began ­closing in stages in the 1980s, taking much of the town’s energy with it. The population here has sagged to about a third of what it was just at its peak.

On Wednesday, US Steel, grappling with imports and competition from China, announced layoffs for 25 percent its North American nonunion workforce. The company did not say how many of those jobs would come from Pittsburgh, where it has 4,200 employees.

In Trump’s rally speeches and in television appearances, he chastises American leaders for how they have handled trade. He vows to impose steep tariffs on China and Mexico, despite warnings that they would ignite damaging trade wars. And he’s not alone: Both Senator Ted Cruz of Texas and Governor John Kasich of Ohio have stoked populist concerns mounting in former manufacturing towns.

On the Democratic side, Senator Bernie Sanders of Vermont has given former secretary of state Hillary Clinton a far bigger challenge than expected, in part due to his message that the American worker has gotten a raw deal through trade pacts like the North American Free Trade Agreement.

Clinton has fended off criticism from Sanders for statements she made in support of the Trans Pacific Partnership, a trade deal Clinton says she now opposes.

Trade overlaps with a number of other major themes of the 2016 campaign, all of which Trump has capitalized upon to varying degrees. They include fear of illegal immigration and a sense that the traditional Judeo-Christian identity of the country has changed; concerns that American strength around the world has been sapped; and rage toward elites who seem to suffer little amid this unease.

“Obviously, I’m not rich or making millions of dollars, but I just like to see what [politicians are] going to do for middle-class people,’’ said Melissa Day, 34, a minimum-wage bartender at Demenel’s in Aliquippa, where a sizeable fried fish sandwich fetches $6 and fresh cigarette smoke hangs in the air. Trump, she said, “doesn’t hold any punches, which is a good thing.’’

Carolyn Renninger, an electric controls manufacturing executive who has previously backed Republicans, said that Trump’s “presidential aura needs to be refined a little bit’’ but that she hopes he “will make it.’’

“I have been very unhappy with the Republican establishment,’’ Renninger said in nearby Ambridge, adding that she likes Sanders but not his politics. “I’m tired. We’re disgusted with it.’’

In Aliquippa, the middle class has largely vanished. After J&L sold the steel mills to LTV Corp. in 1984, tax revenues dropped, draining government budgets. In 2000, LTV sold the tin mill, the only part of Aliquippa Works still functioning, to US Steel, which closed it and laid off the workers.

The population exodus drained life from community institutions. The Pittsburgh Archdiocese closed the town’s remaining parish school in 2009.

It was places like Aliquippa that then-Senator Obama was describing eight years ago, weeks before losing the state primary to then-Senator Clinton. In a closed-door fund-raiser, Obama recounted “small towns in Pennsylvania’’ where “they get bitter, they cling to guns or religion or antipathy toward people who aren’t like them or anti-immigrant sentiment or anti-trade sentiment.’’

After nearly two full Obama terms, little has changed in either the economic or the political conditions in Aliquippa.

Here, they talk brightly about the undefeated state championship season their high school boys’ basketball team completed last month. And city officials speak optimistically of the Marcellus Shale natural gas industry, hoping it could spark an economic revival and reverse the damage done by the area’s de-industrialization.

But locals are less sanguine about the presidential race.

“I don’t think it matters who wins,’’ said Yancy Velez, a 30-year-old carpet installer lunching with his brother on gyros and grape leaves at Papa Duke’s Paris Grill. “No matter who wins, it’s not going to be the people.’’

Jim O’Sullivan can be reached at jim.osullivan@globe.com. Follow him on Twitter @JOSreports.