“As history, precedent, and practice demonstrate, it is plainly permissible for jurisdictions to measure equalization by the total population of state and local legislative districts,” she wrote. She was joined by Chief Justice John G. Roberts Jr. and Justices Anthony M. Kennedy, Stephen G. Breyer, Sonia Sotomayor and Elena Kagan. Justices Clarence Thomas and Samuel A. Alito Jr. agreed with the outcome but filed separate concurrences.
Currently, all states — with some minor variations — use total population for redistricting, and it is required for congressional districts. The general population contains millions of people who aren’t eligible to vote: children, legal and undocumented immigrants, prisoners, and those who are disenfranchised. Except for prisoners, they are largely concentrated in urban areas.
The court did not rule on a separate question: whether it would be constitutional for states to use the voter-eligible population for reapportionment if they chose to do so. The Obama administration argued that it would not be constitutional.''