Skip to content

Will Population Changes in Pennsylvania Since 2020 Help Republicans?

Uneven patterns of migration between counties could make the 2024 presidential campaign even more closely contested than it was in 2020.

Berwood A Yost
3 min read

Dear Readers,

Today we review newly released Census data about county-level population changes in Pennsylvania. It is possible that the uneven patterns of in- and out-migration between counties could work to make what was a closely contested 2020 presidential campaign even closer in 2024.

We plan to release our next statewide poll on April 4, so please keep an eye out for those results.

Thank you for reading,

Berwood Yost

The Census Bureau's release of changes to county-level population since the 2020 presidential election shows that the state's population declined slightly, losing about 41,000 people from the 13.0 million residents counted during the 2020 census. They also show that uneven patterns of in- and out-migration could work to make what was a closely contested 2020 presidential campaign even closer in 2024.

The Political Geography of Population Gains and Losses since 2020

The aggregate population of the counties that voted for Donald Trump in 2020 increased by roughly 4,500 people, while the aggregate population in the 13 counties that supported President Biden declined by more than 45,000 people. The main reason for the larger population decline in Biden counties is because Allegheny County and Philadelphia had the largest net population losses, losing approximately 26,000 and 53,000 residents, respectively. Chester and Montgomery counties, despite having the largest population gains in the state, with close to 15,000 and 12,000 more residents in 2023, could not offset the large losses in the state's two major urban centers.

Table 1 provides more detail about the components of population change, aggregated by 2020 presidential winner. While natural population decline resulting from births and deaths was far larger in the Trump counties, migration into those counties from other parts of the state and nation helped them have a net positive population gain. The counties won in 2020 by President Biden, on the other hand, suffered the most from domestic out-migration.[1] In fact, the population decline in Biden counties would be almost twice as large without international migration, which is of no benefit in electoral politics since only citizens can vote. 

Table 1. Components of Population Change in Pennsylvania Counties by 2020 Presidential Election Winner



Won Counties


Won Counties



Total Births




Total Deaths




Total Births and Deaths




Total International Migration




Total Domestic Migration




Total Migration




Total Population Change




Total Population 2023




Around half of the residents who leave Allegheny and Philadelphia counties move to another part of the state, according to the Census Bureau.[2] Most of those who left Allegheny County moved to its surrounding counties: Westmoreland, Washington, Beaver, and Butler. The same pattern holds for Philadelphia. The most common destination for in-state movers from Philadelphia are its surrounding counties of Montgomery, Delaware, Chester, and Bucks. What is different, though, is that Philadelphia residents moved to places that, like Philadelphia, supported President Biden in 2020, while Allegheny residents moved from a county that supported President Biden to counties that voted in sizable margins against him. Will those Allegheny County movers take their political preferences, which one could assume tilt towards Democrats, with them in 2024, or leave them behind?

Concluding Thoughts

The Pennsylvania counties that Joe Biden won in 2020 represent a majority of the state's population (55.6%) and registered voters (56.9%). The new population estimates do nothing to change that. Still, the population changes that have taken place since 2020 increase the chances that this election will once again be closely contested, in part because the Trump-won counties are so uncompetitive. In 2020, the median margin of victory for Mr. Trump was about 40 points in the 54 counties he carried, compared to a median margin of victory for President Biden of 8 points in the counties he won.

President Biden's poor job approval ratings suggest that he could lose by larger margins in the 2020 Trump counties while his margins narrow in the places he won. His poor job approval ratings alone pose a challenge to his re-election. The population changes that have happened in the state since 2020 do nothing to make this challenge less difficult.

Additional Reading

Prior newsletters have documented the ways that ideological and regional sorting have changed state politics and how the state's political geography has readjusted itself over the past two decades. These readjustments have reduced the political punch of the most urban counties while the state's most rural county types have increased their political punch at the same time as they have moved overwhelmingly toward Republicans.


[1] The total population figure does not equal the sum of total births, deaths and migration because total population change includes a calculation that represents population changes that cannot be attributed to any specific demographic component. See Population Estimates Terms and Definitions at

[2] See the Census Flows tracker for detailed information about in- and out-migration in the United States at

Electoral ContextPolitical Geography

Related Posts

Members Public

2024 Primary Recap: Protest Votes and Other Surprises

An analysis of the unexpected outcomes in the 2024 Pennsylvania primary election. We look at how the protest vote, turnout, and geography influenced the results, and what this might mean for the upcoming general election.

Members Public

Franklin & Marshall Poll Release: April 2024

See where the 2024 presidential race stands in Pennsylvania, and explore voters' positions on marijuana legalization, voting reforms, and democracy, along with their ratings of President Biden, Governor Shapiro, and Senator Casey.

Members Public

Voters' Emotions and the 2024 Presidential Election

How will emotions--both positive and negative--about Biden and Trump affect the 2024 presidential election?