If the court were to find for the plaintiffs – and it seems unlikely that the court would have gratuitously taken up this case, absent a circuit split, if there weren’t some substantial support for the plaintiffs’ position – it would mean that, once again, virtually every legislative and congressional district in the country would have to be redrawn (although this would not, as some have suggested, affect apportionment – i.e. the number of seats allocated to each state). This would occur at a time when Republicans control a record-high number of state legislatures and a majority of state governments. Republicans would be able to update their maps to account for changes in political orientations in their states since the previous round of redistricting.

But this would have implications for Democratic-controlled states as well. Consider that in 2012, counties with high citizen populations were more likely to vote for Mitt Romney (the t-stat is 9.047). Of the 35 states with four or more congressional districts, there was a statistically significant, positive correlation between the share of county residents who were citizens and the share of voters who cast ballots for Mitt Romney in 18 of the states, most of which are among our largest: California, Colorado, Florida, Illinois, Indiana, Kentucky, Maryland, Michigan, Minnesota, Missouri, Nevada, New York, Ohio, Oregon, Pennsylvania, Texas, Virginia and Wisconsin.

By ruling that states had to ensure an equal number of eligible voters in districts — rather than persons — the court would force even Democratic map makers to push districts out of heavily Democratic areas and into Republican areas. Just 78 percent of the residents of heavily Democratic Queens County are citizens, and 83 percent of similarly Democratic Kings County residents are citizens. In neighboring Nassau County, however, 91 percent of residents are citizens. In other words, 20 percent of the population in districts in Queens and Brooklyn would no longer count. To populate these districts, line drawers would be forced to push districts into Nassau County. Democrats would have a choice between weakening the Queens/Brooklyn districts, or making the remaining Nassau County districts more Republican.