WASHINGTON — A closely divided Supreme Court on Tuesday struggled to decide "what kind of democracy people wanted," as Justice Stephen G. Breyer put it during an argument over the meaning of the constitutional principle of "one person one vote."
The court's decision in the case, expected by June, has the potential to shift political power from urban areas to rural ones, a move that would provide a big boost to Republican voters in state legislative races in large parts of the nation.
The basic question in the case, Evenwel v. Abbott, No. 14-940, is who must be counted in creating voting districts: all residents or just eligible voters? Right now, all states and most localities count everyone.
The difference matters because people who are not eligible to vote — children, immigrants here legally who are not citizens, unauthorized immigrants, people disenfranchised for committing felonies, prisoners — are not spread evenly across the country. With the exception of prisoners, they tend to be concentrated in urban areas.
Their presence amplifies the voting power of eligible voters in those areas, usually helping Democrats. Rural areas that lean Republican, by contrast, usually have higher percentages of eligible voters.
The case, a challenge to voting districts for the Texas Senate, was brought by Sue Evenwel and Edward Pfenninger, who asked the court to require states to count eligible voters. They are represented by the Project on Fair Representation, a small conservative advocacy group that has been active in cases concerning race and voting.
The case's partisan overtones were not acknowledged during the argument, but the court's four Democratic appointees asked questions suggesting that they generally favored counting everyone while several of the five Republican appointees said that voter equality was an important interest.
Chief Justice John G. Roberts Jr., for instance, seemed attracted to counting only voters. "It is called 'one person one vote,'" he said. "That seems designed to protect voters."
But Justice Sonia Sotomayor said there were other interests at stake. "There is a voting interest," she said, "but there is also a representation interest." She meant that politicians represent all of their constituents, not just the people who can vote.
Justice Ruth Bader Ginsburg underscored that point by noting that women were counted when districts were drawn long before they gained the right to vote in 1920.
Justice Anthony M. Kennedy seemed to be looking for a middle ground. "You should at least give some consideration to this disparity you have among voters" in different voting districts, he said, adding that it may be possible to achieve both goals. "Why is one option exclusive of the other?" he asked.
Lawyers defending Texas' approach said the resulting districts would be misshapen and drawn without regard to other considerations, including county lines and race.
Ian H. Gershengorn, a deputy U.S. solicitor general, gave an example. "Manhattan has 9 percent children," he said. "Brooklyn has 30 percent. If you have to do both, what you're doing is pairing people from part of Manhattan and pairing them with voters in Brooklyn."
The Constitution requires "counting the whole number of persons in each state" for apportioning seats in the House of Representatives among the states. Justice Elena Kagan said it struck her as unlikely that a different rule should apply for purposes of drawing state districts.
"How you go from that being mandated," she said, referring to counting everyone, "to it being prohibited in the state context is something that I still can't quite work myself around."
The Supreme Court has never definitively ruled on who must be counted. The "one person one vote" principle, rooted in cases from the 1960s that revolutionized democratic representation in the United States, applies to the entire American political system aside from the Senate, where voters from states with small populations have vastly more voting power than those with large ones. Everywhere else, voting districts must have very close to the same populations.
In court papers, Evenwel and Pfenninger said that they lived in "districts among the most overpopulated with eligible voters" and that "there are voters or potential voters in Texas whose Senate votes are worth approximately one and one-half times that of appellants."
Their lawyer, William S. Consovoy, said the case boiled down to a simple proposition. "One person can't be given two votes while their neighbor is given one vote," he said.
Scott A. Keller, Texas' solicitor general, defended his state's decision to count everyone.
"The only question the court has to resolve here is whether the equal protection clause requires every state to change its current practice and use voter population to reapportion," he said. "The answer is no."
But Keller added that the decision of whom to count should be left to the states, leaving open the possibility that states could count only voters. "It's our position," he said, "that we could choose a reliable measure of voting-eligible population without running afoul of the equal protection clause's guaranteed against invidious discrimination."
Gershengorn, representing the federal government, argued in support of Texas, but only to a point. He said the court should not allow states choices beyond counting everybody.
Last year, a three-judge panel of the U.S. District Court in Austin dismissed the case, saying that "the Supreme Court has generally used total population as the metric of comparison." At the same time, the panel said, the Supreme Court has never required any particular standard. The choice, the panel said, belongs to the states.
There are practical problems, many political scientists say, in finding reliable data to count only eligible voters.
Ginsburg mentioned a brief filed by Nathaniel Persily, a political scientist at Stanford Law School, that said there is only one constitutionally required and reliable data set: the census. But the census counts everyone, the brief said, and there are no comparable data for eligible voters.
Short of requiring the government to collect new data, the brief said, a ruling requiring districts to be based on equivalent numbers of eligible voters would be impossible to put in place with anything like the confidence provided by the census.
Consovoy responded that courts have accepted other sources of data to assess other kinds of voting rights violations.
Before the court heard arguments on Tuesday, it issued a unanimous decision in a different election-law case, Shapiro v. McManus, No. 14-990, arising from a challenge to Maryland's 2011 congressional maps. Democrats, the challengers said, had gone to elaborate and unconstitutional lengths to create oddly shaped districts to favor their candidates.
A single federal trial judge dismissed the case, saying it did not present issues serious enough to warrant convening the three-judge court ordinarily required by a federal law in cases concerning apportionment of congressional districts.
"Perhaps petitioners will ultimately fail on the merits," Justice Antonin Scalia wrote for a unanimous court, but the law "entitles them to make their case before a three-judge district court."