President Joe Biden received a "huge red flag" in Pennsylvania, a key swing state in the 2024 presidential election.

New data of voter registration in the Keystone State suggests Republicans are gaining ground against Democrats compared to 2020, though Democrats still have an advantage. Pennsylvania is viewed as one of the handful of battleground states that could determine the outcome of the election between Biden and former President Donald Trump in November.

In 2020, voter registration data showed there were about 800,000 more Democrats than Republicans in Pennsylvania, despite Biden winning the state by just over 80,000 voters (about 1.2 percentage points). As of last month, that number has dropped to only about 375,000, according to the data.

This year alone, 28,382 Democrats have changed their party affiliation to Republican, while only 9,846 Republicans have changed their affiliation to Democrat as of June 24.

Jennie Sweet-Cushman, professor of political science at Chatham University, told Newsweek on Monday that these shifts toward Republicans should be a "huge red flag" for the Biden campaign.

"Seemingly the primary justification for seeking re-election is that he's the best positioned to beat Trump because he's done it before. Unfortunately, in Pennsylvania that was a narrow victory. He'll need to hang on to his voters to do it again and between the deterioration of his support with young voters, the enthusiasm gap, and Republicans rewarming to Trump it is looking more and more challenging that he'll be able to do that," she said.

J. Wesley Leckrone, associate dean of social sciences at Widener University, told Newsweek on Monday that small shifts toward either Biden or Trump could affect the outcome due to the closely divided nature of the state.

"However, this has been a long-term realignment in the state where people's party registration is now more accurately reflecting the way that they vote. The 2024 election in Pennsylvania is going to be won on party turnout and swaying independent voters," he said.

Newsweek reached out to Biden's campaign for comment via email.

The shifts largely reflect trends that have played out nationally over the past few years, with voters living in rural areas changing their party registration from Democrat to Republican. Democrats have gained ground in suburban areas, particularly the counties near Philadelphia.

More rural areas in the western area of the state such as Lawrence, Cambria and Washington counties, saw Republicans make their strongest gains over the past four years, at least in terms of percentages.

It was the Democratic bastion of Philadelphia, however, that saw the largest shift in terms of raw numbers. In the state's largest city, there were 708,103 more Democrats than Republicans in 2020, but only 654,040 in 2024. This means the gap narrowed by 53,463 voters.

Democrats received some better news from suburban areas. In Chester County, there were only 786 more Democrats than Republicans in 2020. By 2024, that number grew to 7,076. In Delaware County, there were 41,631 more Democrats than Republicans in 2020. That number grew to 57,536 this year.

Republicans have also gained ground in the state's most competitive counties that have gone back and forth between supporting Democrats and Republicans.

In Northampton County, Republicans have netted 571 voters in 2024. In Erie County, Republicans have netted 470 voters. These two counties are expected to be closely watched ahead of November, as Biden won Northampton County by only 1,233 votes and Erie County by 1,417 votes in 2020.

Leckrone noted that voters in the western part of the state have "slowly realigned their party registration with the way that they had been voting," and that Democratic realignment in the Philadelphia suburbs occurred much more quickly.

"The current party registration numbers more accurately reflect the way that people have been voting over the last several election cycles. This is evidenced by the fact that Republicans have been competitive in recent state-wide elections even though Democrats had a big voter registration edge," he said.

Sweet-Cushman said voter enthusiasm could also play a role in voters changing their registration.

"Republican voters in Pennsylvania are, on average, more enthusiastic about casting a vote for Trump than Democratic voters are in voting for Biden. This likely means that some of those registrations are those who were leery of Trump in 2020 who are coming back to the party in 2024. Those folks probably never were genuinely Democrats," she said.

Democrats have the "apparatus to be successful" in Pennsylvania due to strong organizing in the state, but lack the messaging, she said. Democrats should focus on policy issues like reproductive rights, environmental protection, or family leave, rather than "trying to enlist them in an existential war for the soul of the country," she added.

Leckrone noted that Democrats should focus on appealing to working, non-college educated voters by emphasizing economic issues.

Recent surveys of Pennsylvania show the race remains tight. An Emerson College poll conducted among 1,000 registered voters from June 13 to June 18 found Trump leading Biden by two points (51 percent to 49 percent). A Redfield & Wilton Strategies poll, conducted among 456 likely voters from June 8 to June 11, found Trump leading by two points (44 percent to 42 percent).