The case concerned the “independent state legislature” theory. It is based on a reading of the Constitution’s Elections Clause, which says, “The times, places and manner of holding elections for senators and representatives shall be prescribed in each state by the legislature thereof.”

Proponents of the strongest form of the theory say this means that no other organs of state government — not courts, not governors, not election administrators, not independent commissions — can alter a legislature’s actions on federal elections.

Chief Justice Roberts rejected that position. “The Elections Clause does not insulate state legislatures from the ordinary exercise of state judicial review,” he wrote.

The ruling soundly dismissed the theory, one that an unusually diverse array of lawyers, judges and scholars across the ideological spectrum viewed as extreme and dangerous. Adopting the theory, they warned, could have profound consequences for nearly every aspect of federal elections, including by erasing safeguards against partisan gerrymandering and curtailing the ability to challenge voting restrictions in state courts.