The U.S. Postal Service is emerging as a pivotal player as the midterm elections approach. President Donald Trump is seeking greater control over mail balloting while a change to how the agency processes mail is leading to criticism that it’s failing to deliver ballots on time.

Trump filed an emergency appeal to the Supreme Court after being told he can't tighten rules for mail-in voting before the midterms.

President Donald Trump asked the Supreme Court to allow the administration to move forward with implementing a sweeping executive order aimed at mail voting — setting up a major…