The U.S. Supreme Court unanimously turned back a legal effort to reinterpret the “one person, one vote” constitutional rule Monday, ruling that states may rely on total population when drawing their legislative districts.

The case, Evenwel v. Abbott, was brought by two Texas voters, Sue Evenwel and Edward Pfenninger, who challenged the apportionment of Texas Senate districts. With the exception of the U.S. Senate, every American legislative body is apportioned by total population under the “one person, one vote” rule first outlined by the Court in the 1960s.

Evenwel and Pfenninger argued that counting non-voters—children, the mentally disabled, disenfranchised prisoners, and non-citizensbroke that rule and diluted their political power in violation of the Fourteenth Amendment’s Equal Protection Cause. Many observers, including my colleague Garrett Epps, noted that Evenwel’s interpretation would redraw the American political map in favor of a whiter, older, and more conservative electorate.

“In agreement with Texas and the United States, we reject appellants’ attempt to locate a voter-equality mandate in the Equal Protection Clause,” Justice Ruth Bader Ginsburg wrote for the majority. “As history, precedent, and practice demonstrate, it is plainly permissible for jurisdictions to measure equalization by the total population of state and local legislative districts.”

The Supreme Court first forced states to draw their legislative districts with roughly equal populations inside them in two landmark decisions: Baker v. Carr in 1962 and Reynolds v. Sims in 1964. The two decisions enshrined the one-person, one-vote rule in American constitutional law.

Transforming those precedents into a requirement that states use eligible voters instead of total population went too far, the Court ruled. “Adopting voter-eligible apportionment as constitutional command would upset a well-functioning approach to districting that all 50 States and countless local jurisdictions have followed for decades, even centuries,” Ginsburg wrote. “Appellants have shown no reason for the Court to disturb this longstanding use of total population.”

But, she noted, Monday’s decision did not address whether states could use voter-eligible apportionment when drawing legislative districts, only ruling that states were not required to do so. In his concurrence with the Court’s judgment, Justice Samuel Alito seized on this distinction to hint that a future case could allow apportionment methods beyond the status quo.

“Whether a State is permitted to use some measure other than total population is an important and sensitive question that we can consider if and when we have before us a state districting plan that, unlike the current Texas plan, uses something other than total population as the basis for equalizing the size of districts,” he wrote.

Justice Clarence Thomas went even further in his own concurrence. “In my view, the majority has failed to provide a sound basis for the one-person, one-vote principle because no such basis exists,” he wrote. Instead of continuing the “misguided search” for one, Thomas instead urged his colleagues to leave the question of apportionment to the states themselves. “There is no single ‘correct’ method of apportioning state legislatures,” he concluded.

Evenwel was the latest brainchild of the Project on Fair Representation, a conservative legal nonprofit that specializes in mounting legal attacks on the legislative pillars of the American civil-rights movement. The group’s highest-profile victory was in 2013 in Shelby County v. Holder, which gutted the Voting Rights Act of 1965. It also filed an amicus brief in Texas Department of Housing and Community Affairs v. Inclusive Communities Project last year, urging the justices to strike down a key part of the Fair Housing Act of 1968. (In the end, Justice Anthony Kennedy joined with the liberals to save it.)

The group is also part of Fisher v. University of Texas this term, which targets affirmative-action policies in higher education. A ruling in that case is still pending.