The Supreme Court on Tuesday rebuffed a legal theory that argued that state legislatures have the authority to set election rules with little oversight from state courts, a major decision that turns away a conservative push to empower state legislatures.

By a 6-3 vote, the court rejected the “independent state legislature” theory in a case about North Carolina’s congressional map. The once-fringe legal theory broadly argued that state courts have little — or no — authority to question state legislatures on election laws for federal contests.

The court’s decision in Moore v. Harper closes the path to what could have been a radical overhaul of America’s election laws.

A particularly robust reading of the theory — which the court turned aside — would have empowered state legislatures to make decisions on all aspects of elections, from congressional lines to how people register to vote and cast a ballot, without any opportunity for challengers to contest those decisions in state courts under state laws or constitutions. Opponents of the theory argued that it could have led to unchecked partisan gerrymandering, and laws that would make it harder for people to vote.

Chief Justice John Roberts wrote the court’s opinion, joined by the three liberal justices, Sonia Sotomayor, Elena Kagan and Ketanji Brown Jackson, along with two conservatives, Brett Kavanaugh and Amy Coney Barrett. Justices Clarence Thomas, Samuel Alito and Neil Gorsuch dissented.