Is the new Pa. congressional map better for Democrats or Republicans? We tested it.
We crunched the numbers to help you better understand the new map and what it means.
Pennsylvania has a new congressional map that will help shape power and representation for years to come, both locally and in Washington.
We crunched the numbers to help you better understand the new map and what it means.
That includes looking at the partisan and racial breakdown of each district and how it performs on various tests used to measure maps. Those tests are key: States redraw their congressional districts every decade, and Pennsylvania has a history of partisan gerrymandering — drawing maps to unfairly favor one political party.
And Pennsylvania is losing a congressional seat — from 18 to 17 — because of population changes, raising the stakes as Democrats look to cling to their slim congressional majorities in the midterm elections.
That's why we're focused here on partisan implications, but it's important to remember that mapmaking is about more than red vs. blue. District boundaries determine the representation of different communities, and that's not something we can quantify in the same way.
The new map was one of 13 proposed by parties in a redistricting lawsuit, and it was drawn by a Stanford professor who is a well-known expert on redistricting and political geography.
But the state Supreme Court’s 4-3 ruling to choose it drew immediate criticism from Republicans, because the map was submitted by a group of Democratic voters who were supported by a national Democratic redistricting group and represented by a national Democratic lawyer. Republicans also denounced the court for not choosing the map the Republican-controlled legislature had passed. That map, which Democratic Gov. Tom Wolf had vetoed as a partisan gerrymander, was recommended by a lower-court judge.
To better understand the new map the Supreme Court chose, The Inquirer partnered with the nonpartisan Princeton Gerrymandering Project to analyze the districts. Here's what we found, starting with the question everyone’s asking: How Democratic or Republican is each seat?
The current map favorsneither partywith a9–9split, based on the most recent two presidential elections.There are5 strong Democratic districtsand7 strong Republican districts.The remaining6 districts are more competitive, with each side winning between 45% and 55% of votes. Of those,4 districts lean Democratand2 districts lean Republican.
The new map favorsRepublicanswith a8–9split, based on the most recent two presidential elections.There are5 strong Democratic districtsand6 strong Republican districts.The remaining6 districts are more competitive, with each side winning between 45% and 55% of votes. Of those,3 districts lean Democratand3 districts lean Republican.
Our analysis measures the partisan lean of districts based on the average of the two-party presidential vote shares from 2016, when Donald Trump defeated Hillary Clinton by less than 1% of the vote, and 2020, when Joe Biden beat Trump by slightly more than 1%.
That's an imperfect metric, partly because past elections don't always neatly predict future ones, and Pennsylvania voters have a history of ticket-splitting — casting ballots for candidates from different parties in different races. There are Democrats who win in Republican districts, and vice versa.
To help account for that, the analysis uses 55% as the cut-off for what defines a "strong” district. Districts where the presidential victor won 50-55% of the two-party vote are categorized as “leaning” toward that party. And districts that would count as strongly Democratic or Republican are automatically recategorized as leaning blue or red if the district has voted for the other party in either of the last two U.S. Senate races.
The table of recent statewide election results shows how competitive districts can flip in different races and in different years, especially in wave elections like 2018.
So while imperfect, our analysis still gives a pretty good sense of the underlying partisanship in each district. Just don’t treat it as an explicit prediction — especially for competitive seats, in wave elections, and in specific or unusual circumstances.
How red or blue the district is sets the playing field, but candidates, campaigns, the broader national, local political environment, and other factors also matter.
How do Democrats and Republicans cluster across Pennsylvania?
In 2018, the Pennsylvania Supreme Court threw out a congressional map drawn in 2011, saying it was so skewed toward Republicans it violated the state Constitution’s guarantee that “elections shall be free and equal.” The court imposed the current map, under which nine Democrats and nine Republicans represent the state in the U.S. House.
An even split is no longer possible now that Pennsylvania is losing a seat, and there’s an important caveat to keep in mind: Even though voters are about equally divided between Democrats and Republicans, they aren't evenly distributed across the state.
Cities tend to be heavily packed with Democrats, suburbs get more and more politically divided farther out from cities, and rural areas are dominated by Republicans. So in densely populated Southeastern Pennsylvania, for example, Philadelphia and its suburbs make up a huge chunk of Democratic votes.
Here’s how that looked in the 2020 election, with one blue dot for every 10 Biden votes and one red dot for every 10 Trump votes:
Pennsylvania’s political geography, then, inherently favors Republicans to a degree, because Democrats cluster in cities and dense suburbs.
The question is how much the state’s political geography favors Republicans: What’s the range of blue and red seats?
Our partners at the Princeton Gerrymandering Project had computers draw one million maps to find out.
Given a starting map and a set of limitations — such as an acceptable range for the population of each district — they had computers randomly draw maps over and over, measuring the partisan composition of each one and counting the number of blue and red districts.
What emerges is a clear picture of the range of safe and competitive seats.
Out of one million computer-drawn maps, the base partisanship of the districts almost always favored Republicans. Pennsylvania’s political geography, sliced into 17 districts, naturally supports about six to seven Democratic seats and 10 or 11 Republican seats.
Of course, some seats are safe, while others are more competitive. Using the two-party vote share based on the average 2016 and 2020 presidential vote, the most common outcomes were between five and six “safe” seats that are at least 55% Democratic, and up to two seats that lean Democratic with 50-55% of the vote.
Specifically, the single most common Democratic outcome was five safe Democratic seats and one Democratic-leaning seat, which occurred 32% of the time. That was followed by five safe and two lean seats (29% of the time) and six safe and no lean seats (15%).
Republicans, on the other hand, have more safe seats, between seven and nine, along with one to four seats that lean Republican.
The single most common outcome for Republicans was to have eight safe Republican seats and three Republican-leaning seats, which occurred in 27% of the computer-drawn maps. That was followed by eight safe and two lean seats (22%) and nine safe and one lean (12%).
So Pennsylvania’s underlying political geography does favor Republicans. Keep that in mind when considering the various metrics for how skewed or fair a map is.
And remember: The base partisanship only tells you so much.
Factors like incumbency, individual candidates and campaigns, the national and local political climate, and voting access matter, too.
What would happen in a 50-50 election?
Pennsylvania is a very evenly divided state, but what happens in a hypothetical election when the statewide vote is exactly split between the two parties? That's one easy way to test a map: If one party wins a large majority of the seats in a 50-50 election, that's a clear sign the map is skewed to its advantage.
The average statewide vote share from the 2016 and 2020 presidential elections was 50.1% Democratic — reflecting that Biden’s victory was slightly larger than Trump’s. That means it was 0.1 percentage points away from a perfectly split election. So we simply shifted each district’s votes toward Republicans by 0.1 percentage points to see how many districts each party would win in a 50-50 election, based purely on partisanship.
Under the new map, Republicans would win nine districts, or 52.9%, a “partisan bias” of 2.9%. That favors Republicans less than the current nine-nine congressional map’s 5.5% partisan bias, and far less than the 2011 map’s 11%.
How uneven is the distribution of voters?
It might sound odd, but if you’re a mapmaker trying to help your party, you don’t want to win districts too handily. Instead, you want to maximize the number of wins by drawing districts that can be safely won by your party — but not so safely that you’re “wasting” votes in a blowout.
To gerrymander, it’s better to win three districts with 60% of the vote than to only win two districts with 90%.
To skew the lines, then, mapmakers have two main methods of gerrymandering. The first is by “packing” the other party’s voters into a small number of districts that they win by a lot, essentially sacrificing those seats. The other method is “cracking” the other party’s voters across multiple districts so they can’t win any of them.
There are several methods to measure maps and identify the kind of skewed vote shares that arise from packing and cracking. Our analysis uses three.
The mean-median difference looks at the difference between a party’s average vote share and median vote share across all the districts. When the difference is large, it means one party is more clustered in its wins than the other party — and potentially being packed and cracked. The mean-median difference in the new map is 0.86%, which means Republicans have a slight advantage that is smaller than the 2011 gerrymander, which had a difference of 3.24%, and the 2018 court-drawn map, which had a difference of 1.16%.
Packed wins, or lopsided wins, is a metric looking at the average margin of Democratic victory and the average margin of Republican victory. If one party’s average margin is much higher, that means its voters are clustered and might signal they’re being intentionally packed. The new map’s difference of 1.23% favors Republicans. Compare that with the 2011 map’s 5.53% skew toward Republicans and the current map’s -1.06%, which slightly favors Democrats.
So when you look at the presidential vote share for the new districts, Democratic seats tend to have either more competitive margins than Republican ones or to be total blowouts with large Democratic margins. Republican seats are more evenly distributed in their margins of victory.
The efficiency gap looks at each district’s “wasted votes” — all the votes cast for the losing candidate, as well as the votes cast for the winner that go beyond the 50% mark needed to win — to see how efficiently both parties’ votes translate to winning seats. The higher the efficiency gap, the more tilted the map is toward one party. The new map’s efficiency gap of 2.67% skews toward Republicans. The 2011 gerrymander had an even stronger pro-Republican efficiency gap of 10.66%, while the current map slightly favors Democrats by 1.06%.
Are the districts sprawling or compact?
Sprawling districts with long tentacles and weird edges can be a sign that district lines were drawn to carefully include some voters and exclude others for partisan reasons.
There are several ways to measure compactness from a zero to 100 scale. Our analysis includes multiple metrics, but we'll focus on just one here, called the Reock score.
Because a circle is the most compact shape, this method draws a circle around each district and looks at how much of the circle is covered by the district. The more compact the shape, the more circular it is, and the closer to 100% of the circle it would cover.
Consider Pennsylvania’s most sprawling and compact districts from the past decade:
We use the average of all the districts’ scores. And because compactness is based only on the shape of districts, not how big or small they are, it’s possible to use the 2011 and 2018 maps for context, even though they had 18 districts, not 17.
The new map has an average Reock score of 45%.
About this story
This article is part of a partnership between The Inquirer and the nonpartisan Princeton Gerrymandering Project to use computational methods to analyze and contextualize political maps. PGP is providing custom data analysis The Inquirer is using in its redistricting coverage. The Inquirer maintains full editorial control of its coverage.
The partisan metrics in this article are based on the average of the two-party 2016 and 2020 presidential vote. Other statewide races were also examined to ensure those scores make sense: U.S. Senate in 2016 and 2018, governor in 2018, and down-ballot statewide row offices in 2020. Results are aggregated into districts from certified precinct results.
Districts are categorized into four groups: “Strong” Democratic seats, Democratic-leaning seats, Republican-leaning seats, and “strong" Republican seats. We used two rules to categorize districts. First, districts that are won by 55% or more of the presidential vote average are categorized as “strong” seats for the winning party, while districts with victories between 50 and 55% are classified as lean seats. We then use the results of the two most recent U.S. Senate races — from 2016, when Republican Pat Toomey won, and 2018, when Democrat Bob Casey won — to reclassify a “strong” seat as a “lean” district if its voters chose the other party’s Senate candidate in either election.
Staff Contributors
- Reporting: Jonathan Lai, Aseem Shukla
- Design and Development: Sam Morris
- Editing: Dan Hirschhorn
- Data analysis: Princeton Gerrymandering Project