Texas and the Obama Administration claim that their way of counting total population must be used because it’s impossible to break down illegal immigrant populations by district. But that’s not true. The Census Bureau estimates the total U.S. illegal immigrant population based on its house-by-house counting. It can certainly apply those estimates by district and state.

The real worry is political: If Ms. Evenwel prevails, legislative districts will have to be reworked in Texas, and presumably in much of the U.S. too. The next legal challenge would be to Congressional districts, which could mean a re-allocation of seats in the House of Representatives. States like Florida, California, New York, Arizona and Texas, with large illegal populations, could lose House seats.

But then that’s democracy—and the Constitution. The Supreme Court shouldn’t fail to apply its bedrock principles because it might disrupt current officeholders. The issue is whether the Justices are going to apply their precedents to new circumstances, or are they going to implicitly overturn Reynolds because it’s too politically inconvenient?