The court, in a 5-4 ruling, rejected a Republican challenge to a Mississippi law that allows mail-in ballots to be tallied if they are postmarked Election Day and arrive within five business days after the day of the vote.

Trump has been a vocal critic of mail-in ballots, falsely claiming that they are subject to fraud and contributed to his 2020 election loss to Democrat Joe Biden.

The Republican president signed an executive order in March seeking to tighten rules on mail-in voting but it has been blocked by the lower courts.

In a post on Truth Social, Trump described the court’s ruling on mail-in voting as a “tremendous loss” for “voter’s rights” and urged Congress to pass a more far-reaching set of voting restrictions called the “SAVE America” act.

In addition to requiring a photo ID to cast a ballot, the SAVE bill, which has stalled in the legislature, would require proof of citizenship to register to vote – a demand that experts say would push millions of people without passports or birth certificates from being able to participate.

The case decided by the conservative-dominated Supreme Court on Monday involved a challenge to the Mississippi law by the Republican National Committee (RNC). Around 30 states allow ballots from some absentee voters received after Election Day to be counted.

Chief Justice John Roberts and Justice Amy Coney Barrett, both conservatives, joined the three liberal justices on the top court in voting to uphold the Mississippi law.

Under the US Constitution, states retain broad control over the administration of elections.

“Federal law dictates when ballots must be cast, state law governs when they must be received,” said Barrett, who wrote the majority opinion.

“Federal election-day statutes do not prevent Mississippi from counting absentee ballots postmarked by election day but received up to five days thereafter,” Barrett wrote. “Nothing in the federal election-day statutes requires ballots to be received by election day.”

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

Polls show that Trump’s Republican Party faces a serious threat of losing its narrow control of Congress in the midterm elections in November, particularly in the House of Representatives, the lower chamber.

If Democrats win, they have signalled they will block Trump’s agenda and could even move to impeach him.

Democratic Senate minority leader Chuck Schumer welcomed the Supreme Court decision.

“The Supreme Court just upheld this bedrock American principle: if you cast your ballot on time, your vote will count,” Schumer said in a statement.

“Participation in democracy should never be limited — not by your race, where you live, or how you vote.”