The decision signaled a recognition by the court that despite its disdain for both racial and partisan considerations, neither violates the Constitution's "one person, one vote" principle.
It also represented the justices' third consecutive ruling on political redistricting that has pleased Democrats more than Republicans following a landmark 2013 decision that went the other way -- Chief Justice John Roberts' 5-4 ruling that struck down a key section of the Voting Rights Act.
The new decision upheld state legislative lines drawn by an independent commission created in Arizona to take the job away from the legislators themselves. Just last June, the justices had upheld the commission's creation by a 5-4 vote over the objections of Republican lawmakers who said the Constitution gives that task to state legislatures.