The change would produce a political earthquake. Eligible voters as a group are older (no children under 18, to begin with), wealthier, and more Republican—and, even more important in Texas, whiter and more Anglo—than the population at large. Many people in the Southwest—both legal residents and undocumented immigrants—are not citizens. Under the proposed Evenwel rule, only those eligible to vote count.

The plaintiffs cite two seminal cases, Baker v. Carr and Reynolds v. Sims, which together are considered (in shorthand) to have established a rule that districting must be done on a “one person one vote” rule. “The Court,” the plaintiffs argue, “need not look beyond these seminal decisions to resolve the question presented in Appellants’ favor.”

Indeed, the appellants hope the Court won’t look beyond them—because the proposed rule is anchored in scattered language from those opinions, not in the constitutional principle they drew from. In Baker, the Court held for the first time that a state’s legislative districts, if drawn unequally, could be challenged under the Equal Protection Clause. In Reynolds, the Court for the first time struck down a state legislative-districting plan because it drew districts unequal in population. Chief Justice Earl Warren wrote for the six justices that “the weight of a citizen's vote cannot be made to depend on where he lives.” For this reason, he continued, “Population is, of necessity, the starting point for consideration and the controlling criterion for judgment in legislative apportionment controversies ... We hold that, as a basic constitutional standard, the Equal Protection Clause requires that the seats in both houses of a bicameral state legislature must be apportioned on a population basis.”

In a later case, the Court held that Hawaii could use registered voters rather than raw population as the basis for its districts. The majority reached that conclusion because the state had an unusually high number of transient military personnel and tourists—and “only because on this record it was found to have produced a distribution of legislators not substantially different from that which would have resulted from the use of a permissible population basis.” The Hawaii case, Burns v. Richardson, has been read to mean that states have a choice of basis, as long as its plan remains close to overall population numbers. The plaintiffs want to replace that loose rule with a no-choice “eligible voter” requirement. They want the Court to read some of its previous language aggressively (“the weight of a citizen’s vote”) and to ignore other language (”Population is ... the starting point ... and the controlling criterion”).

One would hardly know it from the appellants’ brief, but the basis of all the Court’s districting cases is the Equal Protection Clause of the Fourteenth Amendment. That provision protects the rights of “persons” within a state’s jurisdiction. The framers of the clause knew the difference between “persons,” “citizens,” and voters: Elsewhere in Section One they guarantee the “privileges or immunities” of “citizens of the United States,” rather than of “persons”; in Section Two they discuss the right to vote of some, but not all, “citizens of the United States.”