In a decision that split the court and prompted dissents from three conservative justices, the high court shut down a challenge from Pennsylvania Republicans who sought to block a state court ruling that allowed the deadline extension. But even the dissenting justices acknowledged the legal questions in the case would not have affected the outcome of the November election.
Republicans argued the extension was improper because it was not approved by the state legislature and they have raised questions about the impact on future elections.
"That decision to rewrite the rules seems to have affected too few ballots to change the outcome of any federal election," Associate Justice Clarence Thomas wrote in a dissent. "But that may not be the case in the future."