Jowei Chen and I have written this column for the Washington Post, exploring the implications of changing the unit of apportionment from all people to adult citizens only. The op-ed is based on our forthcoming article in the California Law Review.

The gerrymandering wars are about to resume. Over the next year, every state in the country will have to redraw its congressional and legislative districts. In anticipation of redistricting, Republicans are eyeing a new tactic: For decades, states have equalized the numbers of people their districts contain. But the GOP is now pushing to equalize districts’ citizen voting-age populations instead. Under this approach, noncitizens and children would be invisible for remapping purposes. Only adult citizens would count. . . .

To find out what would happen if states made the switch, we instructed a computer algorithm to generate millions of statehouse maps for the 10 states with the smallest proportions of adult citizens. The algorithm incorporated line-drawing rules like compactness, respect for county boundaries, and compliance with the Voting Rights Act. But half the maps equalized people, while the other half did conservatives’ bidding by equalizing adult citizens instead.

Our results for minority representation were striking. Across all 10 states, the fraction of districts where minority voters can elect their preferred candidates (usually either Black or Latino, depending on the district’s population) fell by an average of three percentage points when the apportionment base changed from people to adult citizens. In major states such as Arizona, Florida, New York and Texas, this decline exceeded six percentage points. In Texas specifically, roughly 10 minority districts disappeared between the equal-person and the equal-adult-citizen simulations. These districts’ elimination would undo overnight a generation of slow diversification in the Texas Legislature, rendering the body unreflective of the Texas population.

Our partisan findings, however, were considerably less dramatic. Across all 10 states, the share of Republican districts rose by an average of just one percentage point when we switched the unit of apportionment from people to adult citizens. True, Republicans benefited more in a few states, Texas in particular. But in most states we studied, including Arizona, California, Georgia, Illinois and New York, there was effectively no difference in the parties’ fortunes between the equal-person and the equal-adult-citizen simulations. This was because many of the minority districts that vanished from one simulation set to the other remained Democratic. They would typically be represented by non-White Democrats beforehand, and by White Democrats afterward.