The Supreme Court on Monday appeared ready to overturn a Mississippi law that allows mail-in ballots to be counted as long as they are postmarked by, and then received within five business days of, Election Day. After just over two hours of oral argument in Watson v. Republican National Committee, a majority of justices seemed to agree with the challengers – which included the Republican Party of Mississippi and the Libertarian Party of Mississippi – that the Mississippi law conflicts with federal laws that set the Tuesday after the first Monday in November as the “election day.”

Because more than a dozen states have similar laws, the court’s ruling – which is expected by late June or early July – could have significant implications for federal elections, beginning as soon as November.

Mississippi passed the law at the center of the case in 2020, in response to the COVID-19 pandemic. Four years later, the Republican National Committee, the Mississippi Republican Party, a Mississippi voter, and a county election official went to court to challenge the law, as did the Libertarian Party of Mississippi in a separate lawsuit (which was later combined with the Republicans’ lawsuit). They argued that Mississippi’s law clashed with a federal law, enacted by Congress in 1845, that establishes the Tuesday after the first Monday in November as “election day.” In 1872, Congress directed that congressional elections should occur on this day, as well.

A three-judge panel of the U.S. Court of Appeals for the 5th Circuit agreed with the challengers that federal law requires all ballots to be received by Election Day. After the full court of appeals – over a dissent by five judges – rejected the state’s petition to rehear the case, the state went to the Supreme Court, which agreed in November to weigh in.

At Monday’s oral argument, Mississippi Solicitor General Scott Stewart told the justices that states have broad power over elections. Laws like Mississippi’s, he argued, are consistent with federal election laws because voters make their final choices by Election Day.

Paul Clement, representing the challengers, countered that when Congress initially passed the law establishing the Tuesday after the first Monday in November as “Election Day,” the casting of ballots and the state’s receipt of ballots were “so inextricably intertwined” that “no one would have thought of one without the other” – supporting his argument that a ballot is final (and the election therefore occurs) when it is received by election officials.

U.S. Solicitor General D. John Sauer argued on behalf of the Trump administration, which filed a “friend of the court” brief supporting the challengers. Sauer told the court that “Mississippi’s theory of election is so general and permissive that it would authorize statutes that Congress could not possibly have approved in the 19th century.”

Several justices focused on the history of election practices and what it might mean for Congress’ understanding of “Election Day” when it enacted the laws at the center of this case. Clement emphasized the “unbroken historical tradition” for much of the 19th century and early 20th century of requiring ballots to be received (normally through in-person voting) on Election Day.

But the lawyers and justices sparred over the significance of departures from that tradition during the Civil War, when some Union states allowed soldiers to vote from the battlefields. Clement insisted that proxy voting was the most analogous to today’s absentee ballots. Five states, he said, still required ballots to be submitted and received in a soldier’s home state by Election Day. Justice Sonia Sotomayor, one of the justices who was most sympathetic to Mississippi, responded that two states had allowed officers to collect and mail-in ballots for soldiers.

Justice Neil Gorsuch, who appeared considerably less sympathetic to Mississippi, expressed concern that voters could recall or revoke their votes before they were actually counted, so that their final choices would not occur before Election Day. Gorsuch asked Stewart to address a hypothetical scenario in which, after Election Day but before a winner is declared, a candidate is revealed to have been colluding with a foreign power. As a result, Gorsuch posited, some absentee voters could recall their mail-in ballots and switch their votes, changing the outcome of the election.

Justice Ketanji Brown Jackson pushed back, emphasizing that in her view the case was not about either ballot recalls or what the history of election practices might have been. Instead, she stressed, the dispute before the justices was over who decides the date by which ballots must be received, and, in particular, whether Congress has prohibited the states from making those decisions. “We’re trying … to figure out,” she said, “what Congress meant when it included Election Day in its federal statutes.”

Justice Samuel Alito suggested that in defending the law, Mississippi faced “a variety of line-drawing problems” – the idea that the state’s position, if taken to its logical conclusion, could lead to extreme and (at least in Alito’s view) undesirable outcomes. For example, he asked Stewart, how long after Election Day can states count ballots?

Stewart’s answer – that states get to make the initial decision, but Congress can always step in to impose limits – proved unsatisfying to Alito.

But Jackson once again pushed back, telling Alito that line-drawing problems “are only problems to the extent that Congress thought they were problems.” The question before the court, she noted, is whether Congress intended to “cabin” the states’ decisions regarding Election Day. And indeed, she said, several federal laws – such as those governing voting for military and overseas voters – indicate that Congress intended to incorporate state laws establishing post-election ballot-receipt deadlines into federal law.

Jackson later noted that Congress is currently considering a bill that would prohibit states from counting ballots received after Election Day. The fact that it believes such legislation is necessary, she posited, indicates that Congress believes that federal law currently permits laws like Mississippi’s.

Justice Elena Kagan echoed Jackson’s thinking. She observed that a 2022 law intended to clarify the process for casting and counting of presidential electors, the Electoral Count Reform Act, specifically refers to “the period of voting.” The use of that phrase, she told Clement, implied that Congress is “fine” with states having a “period” for voting, rather than a single day.

Clement answered that the phrase “period of voting” was intended to refer to early voting. But that answer seemed to create some difficulty for the challengers, as various justices pressed both Sauer and Clement about why, under their position, the statute would allow early voting (which was also not used in early U.S. history) but preclude ballots received after Election Day.

Clement told the court that early voting does not “vitiate the whole idea of an Election Day” in the same way that counting ballots received after Election Day does. And in particular, he emphasized, it does not raise the same concerns about fraud – which were at the core of Congress’ motives in passing the law at the center of the case in the first place.

Justice Brett Kavanaugh had a practical question for Clement. If the court were to rule for the challengers in a decision issued in June, Kavanaugh queried, would it be too late to implement that decision for the 2026 elections?

Clement responded that it would not be. Under federal law, he noted, absentee ballots must go out to military and overseas voters 45 days before the general election in November – which would mean that states would have to mail them in mid-September.

As Kavanaugh’s question suggests, a decision in the case is expected by late June or early July.