In a narrow 5-4 decision, the high court upheld a Mississippi law allowing election officials to count mail-in ballots that arrive after Election Day, as long as they meet state requirements. The justices found nothing in federal law dictating when ballots must be counted.
Mail-in voting rules differ by state. In Illinois, ballots are counted as long as they are postmarked by Election Day and received within 14 days. In Mississippi, the grace period is five days. Republicans had challenged Mississippi's law, arguing that ballots should only be counted if they are received by Election Day.
"The argument was that this federal statute created a day, and the court said it doesn't. It creates a day by which you have to cast your ballot," said Ed Yohnka, with the American Civil Liberties Union of Illinois.
The court's decision affirms that election administration remains largely under state and local control.
"The important thing here is elections according (to) our constitution are functions of state and local government, not of the White House," Yohnka said.
The ruling marks a setback for Trump, who has repeatedly claimed mail-in voting leads to election fraud.
"The ruling that a lot of people were waiting for is detrimental to honest elections," Trump said.
Despite those claims, millions of voters from both parties used mail-in ballots in the 2024 presidential election. Trump himself mailed a ballot in a recent Florida election.
Voting advocates say the method remains secure and expands access, particularly for certain groups. The League of Women Voters points to its benefits for military members, people with disabilities, elderly voters and those living in rural areas. The organization also says grace periods are necessary to account for potential postal service delays.
"It has become much, much more popular. I think it gives people the opportunity to vote when maybe other times they would not have been given the opportunity," said Barbara Kwiatkowsky of the League of Women Voters of Illinois.
Trump is now calling on Congress to pass his proposed SAVE America Act. The measure would allow mail-in ballots for certain groups while requiring voters to provide photo identification and proof of citizenship. At this point, the proposal does not have enough support to pass in the Senate.