It’s time to define what the meaning of the word “population” is—and how it applies to the drawing of electoral districts.

In 1964, the Supreme Court ruled in Reynolds v. Sims that, under the Equal Protection Clause of the 14th Amendment, state legislative districts have to be about equal in population. The Court also held that population must always be the “controlling consideration” in state redistricting.

Seem straightforward? Not so fast, say two voters from Texas.

A few years ago, Sue Evenwel and Edward Pfenninger, together with the Austin-based Project on Fair Representation, sued the State of Texas over an implemented redistricting plan that redrew state senate districts based on total population, as opposed to voting population. They argue that such a standard dilutes the voting power of registered voters living in areas heavily populated with nonvoters and illegal immigrants, thus violating the principle of “one person, one vote.”

The Texas Tribune explains:

At the heart of their argument is the disparity between the number of eligible voters in rural and urban areas. For example, the project has noted that Evenwel comes from a mostly rural district with about 584,000 voters, while an adjacent, mostly urban district has just 372,000. The difference, according to the plaintiffs, means the urban voters have more influence.

The Project on Fair Representation, which is helping the plaintiffs in Evenwel v. Abbott, has a history of fighting race-based rules, including a former applicant’s challenge to the University of Texas at Austin’s affirmative action policies. In a statement Wednesday, Edward Blum, the head of the group, said the redistricting case “presents the Court with the opportunity to restore the important principle of one-person, one-vote to the citizens of Texas and elsewhere.”

Texas Attorney General Ken Paxton has focused his response on whether or not the Constitution actually requires the result Evenwel is looking for:

“Plaintiffs cite no case in which a court has accepted their claim that the Constitution compels states to apportion their legislative districts based on voter population as opposed to or in addition to total population,” Texas Attorney General Ken Paxton wrote in the state’s response.

“And multiple precedents from this court confirm that total population is a permissible apportionment base under the Equal Protection Clause.”

If the Court sides with Evenwel, Texas could see a power shift out of urban areas and into more rural areas, which has the potential to bode well for conservative activists and legislators looking to maintain their hold on the state legislature and executive agencies. The question is, of course, as Paxton asks: does the Constitution require this? Or does it just sound like a good idea?

It’s an important distinction.

We’ll be following Evenwel v. Abbott as the Supremes put both sides through their paces. The case is set to be argued in the fall.