Bernie Grofman and Jonathan Cervas for the Monkey Cage:

A map that meets all these criteria will probably reduce the Republican Party’s advantage in the state’s congressional races. Although the two parties poll more or less equally statewide, Republicans hold 13 of the state’s 18 seats in Congress. As is true nationwide, Democrats are clustered around the major cities — Philadelphia and Pittsburgh — while Republicans are spread more broadly around the state. A new, nonpartisan district map would almost certainly give Pennsylvania Democrats a better chance at holding more House seats.

But eliminating gerrymandering is not as straightforward as you might think

There are at least two important legal issues the court would have to tackle in a new map.

First, partisan gerrymandering is like cancer: Sometimes the signs are obvious, but sometimes no signs can be seen. Districts that are bizarrely drawn and unnecessarily fragment existing political boundaries such as townships and counties almost always indicate partisan gerrymandering. But sometimes districts satisfy good-government criteria on their face and nonetheless have egregious partisan intent and effects. The court will have to watch out for such maps, lest it approve what we might call “stealth” gerrymandering.

With that in mind, the Pennsylvania Supreme Court needs to decide whether to evaluate the likely political consequences of proposed plans to prevent such “stealth” gerrymanders. Political science can pretty reliably tell us the likely results of a potential district — understanding, of course, that competitive districts can shift from one election cycle to another, depending on the electoral tides — and understanding that incumbents can be favored beyond the party’s baseline support.

Should redistricting be “neutral” or “fair”?

Second, the court will need to ask whether it is looking for a “neutral” plan or a “fair” plan. “Neutral” treatment involves applying good-government criteria to a map without considering the partisan consequences. A “fair” map is drawn to try to keep the partisan results — in this case, the congressional delegation — more or less in line with the state’s partisan leanings. In other words, if roughly 50 percent of the state population favored party X, a “fair” map would result in party X holding roughly 50 percent of the state’s congressional seats. But if supporters of party X were concentrated in just one or two parts of the state, a “neutral” plan might result in party X holding well under 50 percent of the state’s seats.