Pennsylvania’s new congressional district map, released Monday by the state Supreme Court, is sure to improve Democrats’ electoral outlook in the state. Over the long term, Democrats can expect to occupy one to two additional seats compared with the current map, according to a FiveThirtyEight analysis. (The state’s congressional delegation currently has 12 Republicans and five Democrats. One seat is vacant.)
The court ordered that the map be redrawn after finding that the current one, which was enacted by the Republican state legislature in 2011, was a partisan gerrymander and violated the state’s constitution. (Republicans were given a chance to submit a substitute plan — which they did. And the Democratic governor, Tom Wolf, was given a chance to reject the plan — which he did.) The map submitted by Republicans probably would have benefited them less than the current map does, but it would still have been better for the GOP than what would be expected based on the partisan makeup of the state. Because the legislature and the governor couldn’t come to an agreement, the court stepped in.
Compared with the current map, the new one could easily be mistaken for a Democratic gerrymander. In reality, it gets much closer to matching the political makeup of Pennsylvania’s electorate,1 which is about evenly divided. (President Trump carried the Keystone State by less than 1 percentage point in 2016, for example.) The new map also splits fewer municipalities and has districts that are more compact than the current one.
Earlier this year, FiveThirtyEight presented seven alternatives to the current congressional maps of Pennsylvania and every other state, each using a different set of criteria. (One prioritized creating competitive districts, for example; another tried to maximize the number of majority-minority districts.) In addition to estimating the electoral implications of each map, we used other measurements to compare them. The goal was to show how different priorities in drawing district lines are sometimes in tension, and you can see that in the new Pennsylvania map.
|Efficiency gap||Competitive districts||Majority-nonwhite districts|
|Competitive||D+6||Compact (algorithmic)||4||GOP gerrymander||2|
|Compact (borders)||R+9||Majority minority||4||Competitive||2|
|Compact (algorithmic)||R+11||Compact (borders)||4||Court-drawn||2|
|Current||R+18||Dem. gerrymander||2||Dem. gerrymander||1|
|GOP gerrymander||R+21||GOP gerrymander||0||Compact (borders)||1|
|County splits||Compactness rank|
|Compact (borders)||17||Compact (borders)||1|
|Majority minority||22||Compact (algorithmic)||3|
|GOP gerrymander||30||GOP gerrymander||4|
|Dem. gerrymander||46||Dem. gerrymander||7|
Using the new map, we would expect Democrats to win 7.5 of the state’s 18 U.S. House seats over the long term,2 based on a model that assigns win probabilities to each party based only on a district’s partisanship.