The US supreme court is hearing arguments on Monday about whether mail-in ballots can be counted if they arrive after election day, which would affect laws in more than a dozen states during a midterm election year.

The justices are considering Watson v Republican National Committee, a challenge over a Mississippi state law that was brought in 2024 by the Republican party. Mississippi allows mailed ballots to be counted if they arrive within five business days of election day, so long as they were postmarked by election day. Fourteen states, Washington DC and three US territories have similar laws that allow for late-arriving ballots to be counted.

Mississippi, a red state, is defending its ability to set its own procedures for elections against the challenge from the Republican party, which argues that the grace period violates federal laws that set election day for the first Tuesday of November.

The court’s conservative justices questioned the Mississippi solicitor general, Scott G Stewart, over whether Congress intended to outlaw ballots arriving after election day, pointing to all the ways elections have changed in recent decades. They laid out hypothetical situations, including lengthy grace periods, asking what the limits would be to counting ballots after election day.

Liberal justices asked friendlier questions, bringing up other federal laws that acknowledge such grace periods, like the Uniformed and Overseas Citizens Absentee Voting Act.

The RNC lost its initial case in district court, then won in the fifth circuit court of appeals. A host of groups representing voting rights advocates, military voters and overseas voters have filed to support the state’s position in the case, saying that a grace period allows voters with unique burdens to have their ballots counted.

“The logic of the Fifth Circuit’s ruling in this case would upend multiple, long-established state laws that specifically use grace periods to alleviate the unique barriers to voting faced by U.S. military and overseas voters,” an amicus brief from individuals and groups supporting Mississippi’s law says.

National Republicans have struggled with mail voting, a common practice in many states, and the rules around it. Some, including Donald Trump, have called to ban mail voting fully, while other Republicans recognize that their voters use mail voting at high rates as well. Getting rid of these grace periods could inadvertently hurt Republican candidates.