The following analysis is a team effort by the Princeton Gerrymandering Project. John O’Neill drew maps which he, Brian Remlinger, Will Adler, and Madeleine Parker analyzed. For other people’s plans, we cited the work of Brian Amos.

In a previous post, we evaluated the Pennsylvania GOP leaders’ proposal for redrawing the congressional map and found strong signs of partisanship. We got curious: under the limits set by the state Supreme Court, what is the maximum advantage in the other direction, favoring Democrats?

We did this in order to put all the proposals into context. If we know the extreme limit on how favorably a map could treat Democrats while following the Court’s directives, then we can evaluate the partisanship of the many Democratic proposals.

We find that the Court’s simple rules (compactness, equal population, and minimization of county/municipality splitting) may still allow Republicans to commit mischief, but they effectively limit Democratic opportunities to gerrymander. Interestingly, Democratic players didn’t go nearly as far as they could have, but instead drew plans that were at the middle of the range of possibilities.

Here’s a map that is impressively compact:
It is the most Democrat-favoring map we could draw under the court’s criteria. On average, it would elect 9 Democrats, 9 Republicans for an equally split statewide vote (using 2012/2016 presidential vote data), with 7 safe seats for each party and 4 tossup seats where the expected margin between parties would be 10 points or less. Democrats can get to 11 seats if they win all the tossups. That’s still 1-2 seats less lopsided than the Republican proposal in the other direction – and requires a wave election.

As far as literal compliance with “floor criteria” goes, this map splits 12 counties, 14 municipalities, and 11 precincts, equal to or fewer than all of the plans we examined. It also performs as well as or better than the other proposed maps on compactness metrics. There could well be a better map out there by these criteria, but we haven’t seen it.

This table contains an analysis of the map above (“Princeton D compact”) and other maps. Click to enlarge:

Democrats gain the biggest advantage by splitting Philadelphia and Pittsburgh. This is the converse of what we saw in the Republican plan, which packed those cities tightly while splitting other communities such as Reading, Montgomery County, and greater Harrisburg.

It’s possible to squeeze out a little more performance by being less attentive to the compactness metrics. The following map (“Princeton D extreme” in the table above) still makes fewer splits than all the submitted plans. Compared with the top map, it converts one safe Republican seat to a toss-up. It’s still a 9-9 map, but its ceiling performance is 12 Democrats, 6 Republicans.

Assuming that the Republican legislative leaders’ proposed map of February 9 was as favorable to their party as possible, this means that under any nominally Court-compliant map, Republicans will always win at least 6 seats and Democrats will always win at least 6 seats. The remaining 6 seats — a full one-third of them — are limited not by geography, but by the whims of redistricters.

Interestingly, none of the remedial plans submitted to the Court, even those submitted by Democratic politicians, sought to maximize Democratic seats. All submitted plans had between 6 and 8 Democratic seats, with most plans settling on 7 Democratic seats. Basically, everybody but the Republicans played it down the middle.

Why didn’t Democrats go for broke? Maybe they think toss-up seats are flippable in 2018. Maybe they wanted to protect their five incumbents. Maybe they prioritized “softer” criteria like maintaining certain political or economic communities. Or maybe they are simply good-faith actors. Whether the Court picks one of their plans or draws its own, the outcome is highly likely to be more fair than the existing plan.

One lesson here is that the median neutrally-drawn map appears to be about tilted two seats toward the Republicans. This cycle, partisan gerrymandering added two seats on top of that.

Back to our maps…to be clear, we don’t believe that our maps are good simply because they perform well on the metrics. Looking at our map one way, it’s the extreme endpoint of a range of possibilities. Looking at it another way, it shows that it’s possible to attain representational equity even in a state as heterogeneous as Pennsylvania.

Whichever way you look at it, we have shown that the simple rules and traditional redistricting criteria used here by the Court can only do so much to constrain partisan actors. The court’s floor criteria of compactness and no splitting can be useful to prevent the flagrant geographic offenses we saw on both sides of the aisle nationwide in 2011. However, a truly fair process can only result from a nonpartisan process that examines the many tradeoffs in redistricting.

The fine print:

We used 2016 2-way presidential vote to categorize districts as Democratic or Republican. 2016 was probably the high-water mark for Republicans in Pennsylvania, so our map (and the other maps) may be slightly more favorable to Democrats than the data suggests. We did not examine incumbency, nor did we attempt to keep communities together beyond counties or municipalities. The plans above were drawn by John O’Neill, an expert with real life redistricting experience. We can’t rule out that there is some plan much more advantageous to Democrats than what he drew, though we suspect not. A shapefile of our map is available here, and summary statistics of our plan, as well as some others, are here.

We examined the following maps, which are available at the PA Supreme Court website. The analysis here taken from Brian Amos on Twitter, unless otherwise noted. All analysis uses 2016 presidential results:

• PA House Democratic caucus’ plan: 6 safe D, 1 lean D, 2 lean R, 9 safe R

• PA State Senate Democratic caucus’ plan: 6 safe D, 1 lean D, 2 lean R, 9 safe R

• Gov. Wolf’s plan: 5 safe Dem, 2 lean Dem, 2 lean R, 9 safe R

• Lt. Gov. Stackhouse’s plan: 5 safe D, 2 lean D, 2 lean R, 9 safe R

• Petitioner’s plans: 5 safe D, 2-3 lean D, 1-2 lean R, 8-9 safe R

• Republican intervenors’ plan: 5 Dem safe, 1 Dem lean, 2 R lean, 10 R safe

• Republican legislative plan (aka Scarnati and Turzai): 5 safe D, 1 lean D, 3 lean R, 9 safe R (analysis)

• Schneider/Wolf amicus plans: 4-5 Dem safe, 2-3 Dem lean, 3 Rep lean, 8 Rep safe (analysis)