At issue is the principle dating back to the 1960s when the Supreme Court held that state legislative districts must be drawn so they are equal in population. Tuesday, justices dealt with something they've never explicitly answered: whether the doctrine applies to the general population or the voting population.
Civil rights groups are watching the case carefully, fearful that if the Court rules with the plaintiffs, it could potentially shift power from urban areas -- districts that tend to include a higher percentage of individuals not eligible to vote such as non-citizens, released felons and children -- to rural areas that are more likely to favor Republicans.
Justice Ruth Bader Ginsburg pointed out that most jurisdictions have used equal population to draw district boundaries. "So we have the states overwhelmingly -- for half a century -- using population as shown in the census, and now you're saying that they can't do that anymore," she asked lawyers for Sue Evenwel and Edward Pfenninger, two Texans who brought the challenge.
Edward Blum, the director of a conservative group called Project on Fair Representation, is backing the challenge. Blum's group was also behind a 2013 case that invalidated a central provision of the Voting Rights Act as well as a case this term seeking to strike down a race-conscious admissions program at the University of Texas. The plaintiffs argue that their vote is being diluted in relation to voters in other districts and that Texas must look primarily at the total number of eligible voters when it draws district lines.
"This appeal presents a fundamental question," William S. Consovoy, a lawyer for Evenwel, told the justices. "That question is whether the one person, one vote rule affords eligible voters any reasonable protection," he said.
He was pressed by Justice Sonia Sotomayor. "The problem is that what you're forgetting is the dual interest," she said. "There is a voting interest but there is also a representation interest," she said. She also questioned the practical consequences of using data that she said had "almost decisively been proven as being inadequate" to measure eligible voters.
Civil rights groups fear that Latino communities in certain states with nonvoting residents, as well as children and others, would be sharply disadvantaged if the Court were to side with Evenwel. "Drawing districts to equalize people is the only way to ensure that the communities where people live and work are fairly represented in the nation's legislatures," Michael Li, counsel for the Brennan Center's Democracy Program said after oral arguments.
Justice Elena Kagan pointed out that when it comes to the apportionment of House members, total population is used as the metric. "Why would it be the case that the Constitution requires something with respect to one apportionment that it prohibits with respect to another?" she asked Consovoy.
Texas Solicitor General Scott A. Keller argued on behalf of the state, which is defending how the boundaries are drawn. "Texas validly used federal census data to equalize total population," he said, stressing the court should give the states leeway in the area. "It's our position," he said, "that we could choose a reliable measure of voting-eligible population without running afoul of the Equal Protection Clauses' guarantee against invidious discrimination."
"But why is one option exclusive of the other?" Justice Kennedy -- often a key swing vote -- asked. "Why can't you have both? You have population equality and voter equality, both, especially when you have indicated that a voter-based apportionment is valid and serves important purposes?"
At one point Chief Justice John Roberts seemed to stress the importance of voters. "Well, it is called "one person, one vote," that seems to be designed to protect voters," he said. Justice Antonin Scalia, another conservative who is normally an active participant at oral arguments, asked no questions.
The Obama administration sides mostly with Texas in the dispute, arguing that all states use total population data collected through the federal census. And in briefs supporting Texas, civil rights groups and the Democratic National Committee worry that an adverse ruling might negatively impact those who don't vote.
"To reverse course now is to shun precedent and needlessly engineer a plan to bring harm on a specific subset of people; namely communities of color -- including children -- undocumented individuals and families, persons with prior felony convictions and individuals with disabilities," said Penda Hair, co-director of the Advancement Project, in a statement.
Also supporting Texas is Nathaniel Persily of Stanford Law School, who says that if the court were to say that the Constitution requires states to use the voting population, it could unleash a series of questions regarding the reliability of voter lists and surveys. "A national database of eligible voters does not exist and will not exist in the foreseeable future," he said in an amicus brief.
But Ilya Shapiro of the libertarian Cato Institute says times have changed since the court came down with the one-person, one-vote doctrine and that modern immigration patterns have created disparities in voting districts. He supports Evenwel.
"Just as it was intolerable for a rural district with 500 voters to have the same representation in a state legislature as an urban district with 5,000 voters, it's now constitutionally suspect to have that disparity between a heavily (noncitizen) foreign-born district and one with mostly native-born citizens," Shapiro wrote in an essay for Scotusblog.
A three-judge district court ruled against Evenwel and dismissed the case.