Donald Trump signed an executive order directing his administration to compile a national voter file and to restrict the use of mail-in ballots, an unprecedented move that is probably unconstitutional.

The executive order directs the Department of Homeland Security to work with the Social Security Administration to compile a list of verified US citizens who can vote in every state. It also directs the United States Postal Service (USPS) to begin rule-making on a process that would require states to notify the agency of voters who intend to receive a mail-in ballot and prohibit them from receiving one unless they are on a USPS-approved list of eligible voters.

Trump repeated a series of falsehoods about voting before signing the order in the Oval Office on Tuesday, claiming that cheating with mail-in voting was “legendary”. Repeated studies and investigations have shown there is no widespread voter fraud, including fraud through mail-in voting. The president himself voted by mail earlier this month.

“There’s not a single provision in here that will withstand judicial review. This is a wholly unconstitutional EO,” said David Becker, the executive director for the Center for Election Innovation and Research, a non-profit.

The US constitution gives the president no authority over elections and expressly authorizes states to set election rules. Trump signed an executive order last year seeking to impose citizenship requirements on voter registration as well as mail-in voting restrictions on election rules that was struck down.

Several key details remained unclear from the order and underscored why implementing would probably be difficult.

The order requires the administration to provide a verified citizenship list within 60 days of an election. But voters will probably register after that cutoff; federal law says that the earliest a state can cut off voter registration is 30 days before election day. It is unclear what would happen if an eligible voter was not included on the list.

It is also unclear how the Trump administration will know which state voters live in based on DHS and SSA data.

That may be why the justice department has been pressuring states to turn over their voter rolls to the federal government, including sensitive information like the last four digits of their social security number and driver’s license information. The department has not won any of the dozens of lawsuits it has filed seeking the information and has already lost three.

Chuck Schumer, the Democratic leader in the Senate, pledged legal action against the measure: “See you in court. You will lose,” he posted on X.