Washington DC - President Donald Trump called on his fellow Republicans to take action after the Supreme Court issued a damning ruling against his efforts to restrict mail-in voting.

The court ruled in a 5-4 decision on Monday to uphold a Mississippi law that allows election officials a grace period to count mail-in ballots that arrive after Election Day, as long as they were postmarked beforehand.

Justice Amy Coney Barrett wrote the opinion and was the other conservative alongside Justice John Roberts Jr. to vote with the court's three liberals.

The decision also allows 18 other states with similar voter laws to continue at least through the upcoming midterm elections in November.

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The ruling comes as Trump has been aggressively pushing the unfounded claim that US elections are rife with fraud.

In a Truth Social post, Trump described the court's decision as a "tremendous loss," and said it is now "more important than ever" that lawmakers pass the SAVE America Act, which would impose strict new limits – including requiring proof of citizenship when registering or casting a ballot – and significantly restrict mail-in voting.

He insisted the measure would "save our country" from "a powerful Communist Movement taking place in our Country."

Last week, Trump vowed not to sign a bipartisan landmark housing bill until the Senate passes the SAVE Act, arguing that he considers doing so a "national emergency."