ERIE, Pa. — Democrats have been losing voters in Pennsylvania over the past four years, leaving Republicans salivating over the prospect of former President Donald Trump recapturing the biggest swing state in the Nov. 5 election.

Nearly 300,000 registered Democrats have dropped off the rolls since 2020, even as Republicans have netted about 50,000 new registered voters — all in a state President Biden won by roughly 80,000 votes.

The changes come in places like Luzerne County, near Mr. Biden’s hometown of Scranton. In 2020, Democrats held a 20,000 registered vote advantage, but as of last week, they trail the GOP by a few hundred votes. Mr. Trump won the county in 2020 and is expected to steamroll it this year.

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“We have flipped the county red,” said Gene Ziemba, the county’s GOP chairman. “So not only are we going to win, we’re going to smoke them.”

The change in registration is almost all on the Democratic side. In Luzerne County, the party went from about 106,000 voters in 2020 to about 86,000 as of the latest tally on Sept 23. Republicans went from 86,000 to 87,000. Independent and other third-party voters stayed roughly the same.

Statewide, Democrats went from about 4.2 million to 3.9 million. Republicans gained about 50,000 voters to reach nearly 3.6 million. Independents showed the biggest movement, with about 95,000 new voters, climbing past 1.4 million.

Democrats hope a good chunk of those independents are Republicans with Trump fatigue. Others assume it has more to do with a new automatic voter registration law that pushes voters to complete or update their voter registration when they obtain or renew a driver’s license.

“The voter registration gains for Republicans in Pennsylvania, including in some key counties, is, of course, welcome news for the GOP in perhaps the most pivotal state in the cycle,” said Chris Borick, director of the Muhlenberg College Institute of Public Opinion. “The more narrow Democratic advantage in registered voters does ease the pressure on Republicans to overperform among voters their base voters in the state.”

While many analysts are fixated on polling data, the registration numbers offer an alternative look at political strength.

Across the country, that’s generally been good news for the GOP.

It has grown faster than Democrats in swing states of Arizona, North Carolina and Nevada — all places where voters register by party. Michigan, Wisconsin and Georgia, the other battleground states that are up for grabs this year, do not require party registration.

But Democrats celebrated last month when singer Taylor Swift endorsed Vice President Kamala Harris and told fans to register and vote. The government website she linked to recorded hundreds of thousands of visits, though it’s not clear how many followed through to register.

Democrats also celebrated a rise in registrations after Ms. Harris replaced Mr. Biden as Democrats’ candidate.

Republicans attribute their registration numbers in Pennsylvania to a Trump-inspired populist GOP message on the economy, immigration and trade, as well as blowback against Democrats on the cultural front — including transgender issues.

Democrats say they don’t read too much into the numbers.

Working-class voters may just be changing their registration to match how they were already voting when they backed Mr. Trump in 2016 and 2020, said John Cordisco, a former Democratic Party chairman in Bucks County, one of the populous “collar counties” around Philadelphia.

“I don’t know if party registration gives you any idea what the outcome is going to be,” he said.

While Republicans have netted about 13,000 new voters compared to Democrats in Bucks County since 2020, Mr. Cordisco said Democrats still have performed well in local elections. That includes defending their majority on the county commission last year.

“So if they had made such strides, that should not have happened,” Mr. Cordisco said.

Jim Wertz, a Democrat running for state Senate in a district that could swing control of the chamber, gets an up-close look when he’s out knocking on doors in Erie in the northwest corner of the state.

“I think there are more Trump signs out than there were in 2016. But I don’t think it’s changed the aggregate number of Trump voters,” Mr. Wertz said. “I don’t think there’s more people to the party. I think he’s actually lost people.”

Both parties in Erie have lost registered voters. Democrats have lost about 18,000, and Republicans have lost about 7,000.

The rise in voters not affiliated with either major party matches a national trend, and some analysts said it makes it tricky to try to draw any conclusions from the party numbers.

Mr. Cordisco said that, at least in Bucks County, those independents are more likely to vote for Democrats.

“While Pennsylvania Republicans continue to throw up roadblocks to voting, including by trying to ban the use of drop boxes and drop-off sites, Democrats are talking to Pennsylvanians about the issues that matter to them and deploying resources like IWillVote.com to make sure every eligible Pennsylvanian can cast their ballot,” DNC Spokesperson Addy Toevs said in a statement. “We aren’t taking a single vote for granted, and we’ll continue to invest in voter outreach in every county to get out the vote for the Harris-Walz ticket and Democrats up and down the ballot.”

Charlie Gerow, a Harrisburg-based GOP strategist, said Democrats are mistaken if they don’t think the improvement in GOP registration is a problem.

“That is eating a gloomy sandwich on happy bread,” he said, borrowing a phrase from the late Rep. Jack Kemp, a conservative icon.

Mr. Gerow said it is impossible to write off the shift in voter registration solely on the realignment of white working-class voters who were registered Democrats but have been voting Republican.

“There is a wave of new voters and younger voters who are registering in numbers far greater for Republicans than Democrats,” he said.