The US Supreme Court has rejected President Donald Trump's bid to allow the US Postal Service to enforce new restrictions on mail-in voting ahead of November's midterm elections.

The conservative-majority court on Monday refused to lift an injunction imposed by US District Judge Indira Talwani that blocked the measure, dealing a blow to Trump's efforts to tighten mail ballot rules before the vote that will determine which party controls Congress.

Trump signed an executive order in March directing the USPS to deliver ballots only to voters on lists of citizens. Under the measure, states were required to send the Postal Service a list of recipients and use ballot envelopes previously approved by the agency. The Postal Service was permitted to deny ballots that failed to comply with the new standards or were not associated with registered voters on those lists.

The order also required states and local election officials to supply information to an online portal about voters who receive mail ballots, and directed USPS to introduce unique barcodes on postal ballot envelopes.

Judge Talwani imposed an injunction on 4 September, ruling that the measures likely violated the US Constitution and would be impossible to apply given the proximity of the midterm elections. She said implementing the changes so close to November's vote could disenfranchise voters.

A US appeals court subsequently refused to put her order on hold. On Sunday, a second federal judge, Judge Carl Nichols, also ruled against Trump's order.

The Trump administration then filed an emergency request to the Supreme Court, urging it to allow what it called "an important federal policy to protect the mails from being used to commit voter fraud." The administration argued that states "cannot choose to use the federal mails to carry out their elections but then insist that their election-related mail is somehow exempt from the Postal Service's rule-making authority, conferred by Congress."

In late August, the Supreme Court had temporarily allowed Trump to move ahead with the executive order on procedural grounds, without ruling on its legality. In Monday's decision, the court said in an unsigned order that the administration was "unlikely to succeed on the merits" of its challenge.

Justice Brett Kavanaugh, a conservative appointed by Trump, voted with the majority against the administration. He wrote that there was "at least a fair prospect that the final rule falls within the Postal Service's statutory authority," but argued that "applying the rule in the 2026 elections would be arbitrary and capricious." He indicated he could support the policy at a later stage of litigation.

Justices Samuel Alito and Clarence Thomas dissented. Alito wrote that some plaintiffs lacked standing, while the rest were pursuing a claim the court had recently described as a "Hail Mary pass" unlikely to succeed. Both justices argued that "the Postal Service has broad authority to regulate the mail."

Twenty-three states and Washington DC had sued to stop Trump's order, with legal challenges arguing the move violated states' constitutional rights to run their own elections. The postal workers' union also said clerks could not be trained in time for the election.

Critics warned the restrictions could disrupt the delivery of thousands of ballots and disenfranchise groups that rely on mail-in voting. Alabama, North Carolina and Wisconsin have already begun sending ballots ahead of the midterms. About 30 percent of voters, the majority of them Democrats, cast postal ballots in the general election two years ago.

The White House did not immediately comment on Monday's decision.

It is not the first setback for Trump on the issue before the Supreme Court. In June, the justices ruled that states may count ballots postmarked by election day, rejecting the administration's push to block votes from being tallied when received after polls close.

Trump has long cast doubt on the security of mail-in voting, repeating claims of widespread electoral fraud that have been debunked, despite having voted by mail himself, including during the 2024 election.