The Pennsylvania Supreme Court ruled Friday that mail ballots that are improperly dated won't be counted in November.
The decision reverses a lower court ruling from last month that found it was unconstitutional to reject mail ballots that had a missing or incorrect date.
The requirements of ballots in Pennsylvania have long been the subject of litigation, with opponents of the date requirements arguing that they unnecessarily disenfranchise eligible voters.
A group of voting rights advocates went to court in May seeking to block the provision, which required election officials to reject ballots that were incorrectly dated.