GOP Pennsylvania Congressional Map Redraw is More of the Same

Pennsylvania Republicans colored inside the lines in their court-ordered redraw of the state’s congressional district map. The new plan does a much better job at adhering to current municipal boundaries and is more geographically compact than the map used in 2016. But don’t let this map fool you; the PA GOP hasn’t fixed their original wrongdoing. As the old map packed and cracked Democrats and Republicans into districts that gave the former 8 more seats than the latter — in a state that Donald Trump won by just 0.7% in 2016 — so does the new proposal.

The Pennsylvania GOP’s proposed remedy, in short, is just as biased towards Republicans as the old one. And although Pennsylvania’s Democratic Governor Tom Wolf is unlikely to sign onto the plan, it is worth taking a closer look at GOP redistricting efforts in a scenario where they’ve already been caught red-handed doing so.

Thanks to calculations from Nate Cohn of the New York Times Upshot, we have a good idea of what the new maps will look like. Below, I’ve used his data to investigate differences between the maps and gamed out what the shift means for the 2018 election.


On its face, the proposed remedy doesn’t change much for either party. The districts are more compact, sure, and they look more like districts “should” look when overlaid on the county map in the state — two things that could placate advocates of redistricting reform. But the effect of the plan is the same: If the 2016 election were rerun under the new map, Hillary Clinton would still win 6 of Pennsylvania’s 18 seats in the U.S. House of Representatives. The only difference is that the GOP’s new map sacrifices PA-08 for PA-06, switching both by very slim margins. If there were ever a district plan that proved that pretty, compact lines don’t automatically guarantee fair or desirable outcomes, these Pennsylvania maps are that plan.

This story of small changes district-level partisan composition holds true across the map. The biggest change is a 12-point shift for Trump’s victory margin in a seat he won by 21% last year. All other seat-level changes are smaller than 8 points.

This surface-level analysis is already supporting evidence that the PA GOP is continuing to gerrymander congressional districts to its benefit. However, the more nefarious evidence lies beneath the surface.

Under the new map, the median U.S. House seat in Pennsylvania is 1.7% more Republican than the median seat in the last map (when measured in terms of Hillary Clinton’s 2016 vote margin). This is not itself evidence of partisan gerrymandering, but it certainly doesn’t suggest that there has been a conscious effort to move the map more towards an accurate representation of Pennsylvania voters. Since the median seat of the old map was already Trump +9% seat (in a near tossup statewide contest), this shift towards Republicans is even more mischievous.


What about a real measurement of the plan’s bias? One such measure of partisan gerrymandering is the median-mean difference. Since the U.S. House is a single member district system whereby each district gives all of its representation to the winner of the plurality vote in the contest, this measurement helps assess the partisan bias of the geography of a district plan. In tight contests like those in Pennsylvania, the median-mean difference ought to be small, the closer to zero the better.

This measure of partisan bias, while imperfect (and allowing for a certain degree of tilt towards one party as a natural product of geographic districts), is at least useful when used as a relative comparison. The median-mean difference in the old map was a 9% bias towards Republicans. The new map’s has an 11-point advantage in the same direction. Again we uncover evidence that the proposed remedy looks like a better map, but is in fact as bad as (if not worse than) the old plan.

By using 2016 presidential election results to assess partisan bias, this analysis thus far has evaluated the differences of the plans under the same political environment as in November 2016. That is to say that if we assume the same breakdown of presidential vote share in this year’s midterms, we would expect 6 seats for Democrats.

Obviously, that’s not the environment a new map is likely to be tested under in November of this year. Instead, my midterm forecasting model estimates a roughly 10% national margin for Democrats come the sixth of November. What does the number of Democratic seats look like if we shift the national environment by 10%? What if we run this thought exercise under national environments ranging from a tied race to a 15 point Democratic race? The table below answers that question, with the national House margin in the left column, the number of seats Democrats win in PA under the old and new maps in the second and third columns, and the seats lost/gained under the GOP-proposed map in the right column.

Simulated 2018 Midterm Outcomes Under Different PA Congressional Maps
Democratic Seats in PA
Dem. House Margin (%) Old Map New Map Difference
0 5 5 0
1 5 5 0
2 5 6 1
3 5 7 2
4 5 7 2
5 6 7 1
6 7 8 1
7 8 8 0
8 9 8 -1
9 9 8 -1
10 9 8 -1
11 9 9 0
12 9 9 0
13 10 9 -1
14 10 9 -1
15 10 9 -1
This table shows that the new Pennsylvania U.S. House map actually disadvantages Democrats in the most likely outcome of the 2018 midterms, punishing them by one seat in scenarios under which Democrats win by 8 to 11 or more than 13 percentage points. And while the new map does give an extra 2 seats to Democrats in a three or four point race favoring Democrats, current national polling indicates they would have already picked up those seats.

If the PA Republican Party was trying to construct a new map that would be less of a “dumymander” for them (the old plan has them losing 4 seats in the Philadelphia suburbs in an eight-point Democratic national environment) in a November race similar to the one today, they certainly accomplished that. Let me put it this way: Democrats have to win the state by 10%+ to earn an equal number of seats to Republicans in the state. That’s true under both the old and proposed district lines.

The data here are clear: the GOP-proposed PA congressional map redraw is not an adequate fix for their partisan gerrymandering. In fact, it may be an even more egregious infraction than the previous map. Further, if we glance forward toward Election Day 2018 the picture is painted even clearer for us: Pennsylvania Republican legislators are engaging in the same old tricks they did last time, albeit under a prettier facade. It will take a significant departure from these tactics for Pennsylvanians to live under a district plan that produces fair outcomes for both parties.