Washington DC - The US Supreme Court appeared likely on Monday to toss out a state law that allows mail-in ballots received after Election Day to be counted, a case that could have repercussions for November's midterm elections.

The case involves a challenge by the Republican National Committee (RNC) to a law in Mississippi that allows ballots to be tallied if they arrive within five business days after Election Day.

Fourteen US states allow mail-in ballots to be counted after the date of the election if they bear a postmark by Election Day.

Democrats tend to use mail-in ballots more than Republicans, and the practice became more widespread during the Covid pandemic.

Republican President Donald Trump has been a vocal critic of mail-in ballots and has falsely claimed that they contributed to his 2020 election loss to Democrat Joe Biden.

A federal judge ruled in favor of Mississippi, but the ruling was overturned by an appeals court panel, which said ballots must arrive by Election Day to be counted.

The Trump administration has joined the opponents of the Mississippi law, and a ruling by the conservative-dominated Supreme Court could affect the ballot-counting rules in other states.

Paul Clement, a lawyer representing the RNC, said it was problematic to have mail-in ballots received "with or without a postmark in differing ways in differing states."

"That reality gives the lie to the idea that we have a uniform national election day," Clement said.

"You can't have an election unless you receive ballots, and there must be some deadline for ballot receipt," he said. "Nonetheless, Mississippi insists that ballots can trickle in days or even weeks after Election Day."