The Supreme Court is set to hear a challenge to Mississippi's rules for counting mail-in ballots, which could upend the way states handle such ballots. The Republican National Committee and Mississippi’s state GOP are challenging a law that allows state officials to count ballots that have been postmarked by Election Day but arrive up to five business days later. The question before the court is whether federal laws prohibit state lawmakers from creating their own rules to allow for late-arriving ballots to be counted. A ruling by the court could affect similar measures in at least 18 other states and territories. A decision in the case is expected by the end of June or early July. This case is part of a larger political battle over the use of mail-ins, which surged during the COVID-19 pandemic, leading to legal challenges throughout the country.