Pennsylvania is no stranger to partisan gerrymandering disputes. In a blockbuster 2004 case, the Supreme Court declined to strike down the congressional map then in effect. The court didn’t quite hold that the map was lawful; rather, it couldn’t think of a workable standard for evaluating the map’s validity. Another gerrymandering suit is now making its way through the Pennsylvania courts, with a decision expected by the end of the year. But unlike its predecessor, this suit is based on a manageable test as well as a mountain of damning evidence. Perhaps for this reason, it has thoroughly flummoxed the state’s lawyers and experts.
What’s changed since 2004? In recent years, political scientists have developed powerful new tools for evaluating district maps. These tools indicate how skewed a map is in a party’s favor, how likely this skew is to persist in the future, and whether the skew has a legitimate explanation (like a state’s political geography). In the Wisconsin case now pending before the Supreme Court, the plaintiffs used these tools to craft a simple test for gerrymandering:
First, was a map designed to benefit a particular party?
Second, has the map actually exhibited a large and durable tilt in this party’s direction?
Third, is this tilt bigger than we’d expect given the spatial patterns of the state’s voters?