US President Donald Trump has signed an executive order attempting to compile lists of US citizens eligible to vote in each state, though experts say he lacks the authority to do so.

Tuesday's order directs the US Postal Service (USPS) to send ballots only to voters on the lists, created by the Department of Homeland Security and the Social Security Administration.

The Democratic party filed a lawsuit a day later asking a US judge to block Trump's move.

The US Constitution gives states oversight of federal elections, while Congress can make changes.

Judges have blocked a different executive order from Trump on elections that threatened to withhold election funding from certain states that don't follow the president's orders.

The White House said the order directs "the USPS to transmit ballots only to individuals enrolled on a State-specific Mail-in and Absentee Participation List, ensuring that only eligible absentee or mail-in voters receive absentee or mail-in ballots".

The order also requires the Postal Service to make sure all ballots are "placed in secure ballot envelopes marked as Official Election Mail with unique Intelligent Mail barcodes that facilitate tracking".

Trump has promoted claims without evidence of widespread voter fraud through mail-in voting. He said he voted with a mail-in ballot recently in Florida because he is US president.

Rick Hasen, an election law expert at the University of California, Los Angeles, wrote in a blog post that the president "does not have the authority" to implement the president's executive order on balloting by mail.

"To put this in plain terms: the order would use the USPS, which is not under the direct control of the President, to interfere with a state's lawful transmission of ballots," he wrote.

"It seems highly unlikely any of this could be implemented for 2026, even if it were not blocked by courts," Hasen added, referring to the November midterm elections.

The executive order comes as Trump has been pushing Congress to pass the SAVE America Act, which would overhaul US voting rules by requiring proof of US citizenship to vote.