Washington DC - An appeals court on Thursday upheld a lower court's block on President Donald Trump's executive order curbing the use of mail-in ballots in November's midterm elections.

District Judge Indira Talwani issued a preliminary injunction last week barring the US Postal Service from carrying out a plan that critics say would restrict distribution of mail-in ballots.

A three-judge panel of the US Court of Appeals for the First Circuit unanimously upheld Talwani's injunction, which the Trump administration has already appealed to the Supreme Court.

The judges said they agreed with the district court's conclusion that carrying out the executive order "will likely result in the disenfranchisement of millions of voters across the country while providing minimal – if any – gains in combating voter fraud."

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A coalition of Democratic-ruled states sued the administration on the grounds that under the Constitution, states – and not the federal government – retain broad control over elections.

North Carolina began sending mail-in ballots to voters last week, and other states have also begun to do so.

Trump signed an executive order in March seeking to restrict mail-in voting, saying without evidence that it is vulnerable to fraud.

Trump's order would require the compilation of lists of eligible voters and for the USPS to deliver ballots only to voters who are on the lists.

A USPS whistleblower has warned that millions of Americans may not receive ballots because the systems being used by the postal service have been put together in a "sloppy and rushed manner."

Trump has long been a critic of mail-in ballots but has frequently used them himself, including last month when he voted by mail in Florida's Republican primary.

The president has, for years, claimed without evidence that mail-in voting is highly vulnerable to fraud, repeatedly linking it to his false assertion that the 2020 presidential election was stolen from him by Democrat Joe Biden.

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

If Democrats win, they have signaled they would block Trump's agenda and could even move to impeach him for the third time.