On its face, the constitutional doctrine of “one person one vote” seems about as clear as they come: no one’s vote should count more than anyone else’s.

In practice this means all state legislative districts must contain roughly equal numbers of people. The court has never defined who counts as a “person,” but most states count all people living in the district, not only those eligible to vote.

This issue was before the Supreme Court on Tuesday in a dangerous lawsuit from Texas that could upend the widely accepted method of drawing district lines that is based on the number of people, not voters.

Through a series of Supreme Court rulings in the 1960s, the justices developed the one-person-one-vote doctrine in response to persistent abuses of the line-drawing process by white legislators, who diluted the political power of cities that have large populations of African-Americans and other minorities.