Here’s the new Congressional map for Pennsylvania, drawn by advisor to the court Nate Persily. I think it’s a landmark contribution to the understanding of partisan fairness, in one of the most politically heterogeneous states in the Union – “Pennsatucky.”

For anyone who’s drawn such maps, this particular one is a pleasure to look at. It does a great job at compliance. It splits 13 counties, less than any plan offered to the court. The only plan that splits fewer counties or municipalities is the “compact D gerrymander” that the Princeton Gerrymandering Project posted over the weekend. So in terms of basic compliance, it wins.

But more importantly, this map undoes the effects of population clustering. As we showed over the weekend, it is just possible within the Pennsylvania Supreme Court’s rules to allow a plan that reaches partisan balance, i.e. a 9 D, 9 R outcome for a 50-50 statewide vote. However, those rules bias the range of possibilities in favor of Republicans, so that a Republican gerrymander – one of the proposals on the table – was also a possibility.

It appears as if the advisor was either trying to achieve partisan balance or build competitive races (as opposed to, say, picking a plan that was in the middle of the range of possible maps). His map – and the one we gave over the weekend – show that even under the constraint of compactness and not splitting political jurisdictions, it is still possible to build a map that treats both parties approximately equally. Notably, he did so while keeping most population centers together. This map passes all three of our tests of partisan asymmetry.

This outcome fits with an unspoken principle in the court’s decision of partisan symmetry: if the two sides swapped vote totals, then their seat totals should be swapped as well. This fundamental principle of fairness underlies our proposed standards for detecting gerrymandering, as described in our amicus brief in the Whitford case.

It is commonly claimed that partisan bias arises naturally from the way that voters distribute themselves. Certainly such a bias could be visible if one drew thousands (or millions) of maps at random. But districting is not a random process. It is done by human beings, and reflects real-life political and social considerations. In that respect, we shouldn’t care about millions of maps; we should only care about one map, the one that is drawn. Persily has shown that it is possible to maintain all the old principles of geographic contiguity, and still create an overall map that does not give either party an undue advantage. In my view, this is a big deal because it shows that the ideal of partisan symmetry can be a primary criterion for drawing a map.

It seems clear that the Pennsylvania GOP made a serious tactical error. The governor and Democratic legislators had offered a plan that would have retained some GOP advantage. The new plan erases that advantage entirely. In our analysis, based on the 2016 vote the new map produces 5 Democratic districts, 7 Republican districts, and 6 tossup districts. The maximum likely performance by Democrats in 2018 would be 11 seats, a gain of 6 seats over their current representation. That gain would be one-fourth of the 24 seats they need to win control of the House of Representatives. Of course, some of those gains could also go away in a later year that was good for Republicans. That’s the point of electoral competition.