Evenwel was a challenge to Texas’s scheme of districts for its state senate. That map conformed to the basic one person, one vote rule—that is, the districts were divided up so as to produce rough equality (within a deviation of 10 percent or less) of the total population within each district.

Beginning in 1962, a series of landmark cases held that states couldn’t consciously favor some kinds of voters over others. (For a good account of the old regime, see On Democracy’s Doorstep by the historian J. Douglas Smith.) The skew was toward rural districts, regarded as more virtuous than what used to be called “the mob,” meaning people in urban areas.

The language of these cases was sometimes inexact, but the basic rule was clear: within a rough 10 percent margin, districts were to be drawn on the basis of raw population—counting, that is, not only voters but those who, by age, criminal conviction, or citizenship status, could not cast ballots. In only one case did the Court permit any deviation from that principle; that case, Burns v. Richardson, approved a Hawaii map that counted only permanent residents of the state. Hawaii argued that its circumstances were unique: Because it had so many military personnel and tourists, it said, its map would be distorted by counting them. The Court approved that Hawaii map— “only because” it “produced a distribution of legislators not substantially different from that which would have resulted from the use of a permissible population basis.”

The principle rested there until 2013. Then the Project on Fair Representation rushed into federal court to challenge the new Texas senate map. (PFR’s other major recent cases have been Shelby County v. Holder, which gutted the Voting Rights Act’s “preclearance” requirement, and Fisher v. Texas, which seeks to do the same to affirmative action in higher-education admissions.)

PFR cited language from the one person, one vote cases that used the term “voters”; the Equal Protection Clause, they argued, required “citizen voting-age population” (CVAP) rather than total population as the basis for drawing districts. CVAP districting would upend the maps of all 50 states—reducing the clout of cities and boosting suburbs and exurbs, which tend to be whiter, older, and more conservative.

In other words, it would restore much of the effect, if not the justification, of the old, pre-one person, one vote system.

Because it was a challenge to a districting map, federal statutes required it to be heard by a three-judge district court. Late last year, that court rejected the claim, holding that states could use either total population or CVAP—it was “a choice that the Supreme Court has unambiguously left to the states.”

Losers in three-judge cases are entitled to an appeal. Unlike a petition for review, the Supreme Court must decide an appeal, either by hearing argument or by summarily affirming the decision below. By choosing to appeal, PFR unknowingly set the stage for a major defeat.