In Rucho v. Common Cause the Supreme Court held 5-4 that partisan gerrymandering claims are non-justiciable—meaning that a federal court cannot decide them.
Partisan gerrymandering is the practice of drawing legislative districts to benefit one political party. In Davis v. Bandemer (1986) a majority of the Supreme Court held that partisan gerrymandering cases are justiciable. In that case and since then the Court has been unable to lay out a standard for when partisan dominance “is too much.” In Rucho v. Common Cause the Supreme Court announced it will stop trying.
Chief Justice Roberts wrote the majority opinion which his conservative colleagues joined (Justices Thomas, Alito, Gorsuch, and Kavanaugh). Unsurprisingly, the Court emphasized the role of state legislatures in districting: “The Framers were aware of electoral districting problems and considered what to do about them. They settled on a characteristic approach, assigning the issue to the state legislatures, expressly checked and balanced by the Federal Congress.”