McGann, Smith, Latner, and Keena for The Monkey Cage:
In our book, we show that the partisan symmetry standard can be logically derived from the equal treatment of individual voters, based on recent results in social choice theory. In partisan elections, you cannot treat all individual voters equally without treating all parties equally. This means that the party that gets more votes must get more seats. This sounds obvious, but it is precisely what the Supreme Court did not accept in the Vieth case. We show — line by mathematical line — that this logic is inescapable.
If the party that gets the most votes nationally is to get a majority of the seats, then state districting plans also have to treat parties equally — that is, satisfy partisan symmetry. If a redistricting plan violates partisan symmetry, then it necessarily violates the equal treatment of individual voters. Thus, the partisan symmetry standard is indeed judicially discernible from the Constitution.
The fact that partisan symmetry can be derived directly from equal protection is an important advantage over other standards that have been proposed (such as Nicholas Stephanopoulos and Eric McGhee’s “efficiency gap”). Such standards require additional assumptions that would need to be constitutionally justified.