The U.S. Supreme Court appeared poised Monday to strike down a Mississippi law permitting the counting of mail-in ballots that arrive after Election Day, a decision that could invalidate similar provisions in at least 13 other states ahead of the 2026 midterm elections.

During oral arguments in Watson v. Republican National Committee, a majority of justices focused on whether federal statutes defining Election Day as the Tuesday after the first Monday in November require ballots to be received by that date, not just postmarked. The Republican National Committee, joined by the Mississippi Republican Party, a voter, and a county election official, challenged Mississippi's 2020 law enacted amid the COVID-19 pandemic. The law allows ballots postmarked by Election Day to be counted if received within five business days afterward.

Conservative justices voiced strong concerns about the practice. Justice Samuel Alito questioned how long states could extend deadlines, warning of line-drawing problems and noting that 'Election Day' implies a single day for voting, akin to Labor Day or Independence Day. Justice Brett Kavanaugh highlighted risks to voter confidence, stating that if late ballots flipped results after an apparent winner emerged, 'charges of a rigged election could explode.' Justice Neil Gorsuch raised slippery-slope scenarios, asking if states could extend receipt until the next Congress convenes and observing that history shows expansions once allowed.

Chief Justice John Roberts probed the meaning of Election Day, suggesting it encompasses both voting and receipt. Justice Amy Coney Barrett also posed tough questions on federal preemption.

Liberal justices defended state authority over elections. Justice Ketanji Brown Jackson emphasized discerning congressional intent and noted federal laws for military voters incorporate state deadlines. Justice Elena Kagan pointed to the Electoral Count Reform Act's reference to a 'period of voting.' Justice Sonia Sotomayor cited historical precedents like Civil War-era soldier voting.

Mississippi's solicitor general argued voters decide by Election Day via postmark, preserving the conclusive choice. RNC counsel Paul Clement countered that unlimited extensions undermine Election Day's purpose.

The case stems from a lower court ruling favoring challengers, with the 5th Circuit denying rehearing. A Supreme Court decision is expected by late June, potentially reshaping mail voting for federal elections in states like California, New York, and Texas that have comparable grace periods. Proponents of stricter deadlines, including the Trump administration, argue that it bolsters election security and timely results. RNC spokesperson Ally Triolo said the principle ensures 'ballots must be received by Election Day' to avoid confusion.

Such laws exist in 14 states plus the District of Columbia and three territories, affecting millions of voters, including military and overseas. The ruling could prompt states to adjust procedures before November's midterms, amid ongoing debates over mail voting popularized during the pandemic.