New York Times

WASHINGTON — The Supreme Court hears arguments on Tuesday in an important voting rights case. The justices will consider a question so fundamental that it is surprising that the court has never addressed it before. Here is a look at the case, the arguments advanced by the two sides and the practical consequences of the possible rulings.

Q. What is the central issue?

A. The question for the justices in Evenwel v. Abbott, No. 14-940, is what the court meant more than 50 years ago when it established the principle of “one person, one vote.” What is clear is that the principle requires voting districts of very close to equal populations, but the court has never resolved what is the relevant population: Should voting districts have the same number of people, or the same number of eligible voters?

Q. What are the basic arguments?

A. People who say everyone should be counted contend that the Constitution requires or at least permits “representational equality,” with elected officials tending to the interests of the same number of people, whether they are voters or not. People on the opposing side say that voters rather than bystanders must have the determinative voice in a democracy.

Q. Who sued?

A. The case, which challenges voting districts for the Texas Senate, was brought by two voters, Sue Evenwel and Edward Pfenninger. They are represented by the Project on Fair Representation, a small conservative advocacy group that is also behind the challenge to affirmative action in admissions at the University of Texas at Austin that will be heard on Wednesday.

Q. Why does it matter?

A. Most places count everybody. That amplifies the voting power of places with large numbers of residents who cannot vote — including children, immigrants who are here legally but are not citizens, unauthorized immigrants, and people disenfranchised after committing felonies. Those places tend to be urban and to vote Democratic.

A ruling that districts must be based on equal numbers of eligible voters would move political power away from cities and toward older and more homogeneous rural areas that tend to vote for Republicans. But the consequences of such a ruling would not be uniform. For instance, rural areas sometimes gain additional voting power from prisoners incarcerated in the district who cannot vote but who are counted as residents.

Q. Are there practical issues?

A. A brief filed by Nathaniel Persily, a political scientist at Stanford Law School, and several colleagues said there is only one constitutionally required and reliable data set: the census. But the census counts everyone. 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.

Q. Why did the Supreme Court agree to hear the case?

A. The Supreme Court usually gets to choose the cases it will hear. But there are exceptions, like this case. Rulings from some special three-judge federal courts can be appealed directly to the Supreme Court, and the justices are then required to decide the case. Chief Justice John G. Roberts Jr. expressed discomfort with this procedure at an argument in November, saying it sometimes forced the court’s hand.

It is usually safe to presume two things when the court adds a case to its docket: at least four justices wanted to hear it, and the party that filed the appeal is likely to win. Here, the odd way in which the case reached the court means neither presumption applies.