March 23 (UPI) — The U.S. Supreme Court will hear arguments Monday about mail-in voting in Mississippi and whether the state’s ballot deadline is valid.

Mississippi allows mail-in votes to be counted as long as they are postmarked by Election Day, but opponents say that the business of voting should be concluded by the end of the federal election date. The case was filed by the Republican National Committee.

About 30% of voters voted by mail in 2024.

Fourteen states and the District of Columbia have extended deadlines for counting mail-in ballots that are postmarked by Election Day.

“‘Election Day’ is the day when ‘the proverbial ballot box is closed,’ and thus when officials must be in ‘custody’ of every ballot. Because Mississippi’s law kept the ballot box open for five extra days, it was ‘preempted.'” read an amicus brief submitted in the case by the Department of Justice.

A federal district court agreed with Mississippi’s law and said it didn’t find conflict with the federal Election Day statutes. But the U.S. Court of Appeals for the 5th District overturned the decision and said that all ballots must be received by Election Day.

While the states have the right to manage elections, the federal government sets the national election day.

The state says federal law does not ban states from extending the deadline for ballots.

All states require that ballots be postmarked by election day. There are 14 states and the District of Columbia that allow grace periods as long as the postmark is marked as Election Day.

Last year, four states — Ohio, Kansas, North Dakota and Utah — passed laws eliminating grace periods.

Twenty-nine states and the District of Columbia allow military and overseas ballots to be counted if they’re received after election day, according to the National Conference of State Legislatures.