As a perennial swing state, Pennsylvania is often one of the most heavily campaigned states in the country during presidential cycles. In 2016, the Clinton campaign visited the Keystone State 26 times (second only to Florida) and the Trump campaign held events there 28 times (placing it fourth behind Florida, North Carolina, and Ohio).
For such an evenly divided state, whose statewide partisanship is 51% Democrat and 49% Republican, the 2015-2017 map of Pennsylvania’s congressional districts (below) is surprisingly cattywampus. In the lead up to the election, 13 (72%) of Pennsylvania’s 18 congressional districts were held by Republicans. And the 2016 results indicate that the 13(R) 5(D) seat divide will hold for another Congress.
Is it possible that Pennsylvanians greatly prefer Republican candidates when it comes to U.S. House elections but are evenly divided at the presidential level?
The short answer is no.
There is a large disparity between the number of voters who vote for Democratic U.S. House candidates and the number of Democrats who win in Pennsylvania. In 2012, 2.7 of the 5.2 million votes cast for U.S. House went to Democratic candidates, or roughly 52% of the vote, while only 47% of the statewide vote went towards Republican candidates. Despite receiving a majority of the statewide vote, Democrats only won 5 of the 18 House seats, rather than the 9 we might have expected them to win. In 2014, the vote for Democratic House candidates dropped to 46%, and they retained their five seats, which, although slightly more proportional, is roughly three fewer seats than what would be considered reflective of the statewide vote share.
So why is there such distortion in Pennsylvania?
Most likely the distortion is caused by geography. As the map below shows, Pennsylvania is solid blue in its city centers of Philadelphia, Pittsburgh, and Allentown—which is where Democrats win their congressional seats. In many other places, Democrats and Republicans are more evenly distributed, with slight Republican leans in much of the state.
In the city centers, Democratic candidates often win with large proportions of the vote. In the 2012 election, four of the five winning Democrats earned at least 70% of the vote share, and the other Democrat won with 60%. Of the winning Republicans, only one of the 13 won with greater than 65% of the vote, with the majority winning with around 55% of the vote share.
The geographic dispersion of votes in Pennsylvania, and the large margins of victory for Democrats concentrated in a few districts, means that past patterns of distortions will most likely continue well beyond the 2016 cycle.
But Pennsylvania is not the only state with this problem. Many other states, such as Ohio, Florida, North Carolina, and Massachusetts, also have significant misalignment between statewide partisanship and the partisan breakdown of their congressional delegation.