If states are no longer allowed to count mail-in ballots after Election Day, such a move could also lead to the end of early voting.
The issue came up during Monday's Supreme Court hearing over a Mississippi law that allows officials to count ballots postmarked by Election Day that arrive up to five days later.
The case, Watson v. Republican National Committee, could affect voters in 13 other states and the District of Columbia, which have grace periods for ballots cast by mail. An additional 15 states that have more forgiving deadlines for ballots from military and overseas voters also could be impacted.
The Justice Department argued in favor of the RNC's position.
During the hearing, several justices questioned whether votes could be counted before Election Day if they are not allowed to be counted after it.
"We agree with both sides that early voting is still acceptable," Solicitor General John Sauer said.
"There could be a process where ballots are being received earlier, but that ballot box has to close on Election Day," Sauer added.
Chief Justice John Roberts said Sauer was making an arbitrary decision to treat early voting and the late arrival of voters differently.
"I'm not sure I understand how that point is responsive to the point that if the Election Day is the voting and taking that it has to be that day," Roberts said.
"Maybe you're not saying anything other than, well, that's different," Roberts added.
"It's a challenging question," Sauer replied.
RNC lawyer Paul Clement said lawmakers who passed the Election Day law in 1845 would have found it "unthinkable" to count ballots that arrived after Election Day.
"Isn't that true of early voting, too?" Justice Amy Coney Barrett asked.
"Why is that permissible? If we're just going to say historically it just needs to look like it always looked, how come those features fall out?" Barrett added.
Justice Samuel Alito said that because of early voting and ballots being counted after Election Day, "we don't have Election Day anymore."
"We have election month or we have election months," he said.
"Early voting can start a month before the election. The ballot can be received a month after the election," Alito said.
"Some of the briefs have argued that confidence in election outcomes can be seriously undermined if the apparent outcome of the election on the day after the polls close is radically flipped by the acceptance later of a big stash of ballots that flip the election," he added.
A ruling is expected by late June, early enough to govern the counting of ballots in the 2026 midterm congressional elections.
"There's plenty of time," Clement said.
Last year, Trump signed an executive order on elections that aims to require votes to be "cast and received" by Election Day. The order has been blocked in pending court challenges.
The Associated Press contributed to this report.
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