President Donald Trump on Tuesday signed a sweeping executive order attempting a major overhaul of American elections, requiring people to prove their citizenship when they register to vote.

The order — which also includes an array of other changes, from mail-in ballot deadlines to election equipment — could risk disenfranchising tens of millions of Americans. Election law experts questioned whether Trump had the authority to make the changes, saying the order is all but certain to be met with legal challenges.

Federal law currently requires that voters swear under penalty of perjury that they are citizens and eligible to vote when they register, and courts have prevented states from adding documentary proof-of-citizenship requirements for voters in federal races because of such laws.

Trump’s order directs the Election Assistance Commission, an independent, bipartisan commission that supports election officials, to redo its voter registration form and require voters to show U.S. passports or other government ID that shows citizenship to register to vote.

Roughly half of Americans had U.S. passports last year, according to the State Department, and a birth certificate is not listed as an acceptable proof of citizenship under the order. Some of the other eligible ID records Trump’s executive order suggests — like REAL IDs and military identification cards — do not always show citizenship, either.

The order says the actions are designed to prevent noncitizens from voting, though there is no evidence they do in significant numbers in the United States. Voting as a noncitizen is a serious crime that creates a paper trail that must be routinely reviewed by election officials.

Trump has long railed against noncitizen voting as part of his unfounded election fraud claims.

Election experts said they expected the order to be challenged in court.

“A whole lot of this is illegal,” said Sean Morales-Doyle, director of the Voting Rights Program at the Brennan Center for Justice at NYU School of Law.

Presidents are not granted authority over the Election Assistance Commission or elections, Morales-Doyle and other election law experts have pointed out in the hours since the order was signed.

“The president’s got almost no power over federal elections,” said Justin Levitt, a constitutional law scholar at Loyola Law School. “As the senior policy adviser for democracy and voting rights in the last administration, one of the things that was very clear is how little power the president has over federal elections — by design.”

The Constitution gives Congress and the states power to regulate the “times, places and manner of holding elections.”

Republicans in Congress have introduced a documentary proof-of-citizenship requirement bill, called the SAVE Act. It would allow voters to use their birth certificates to prove their citizenship, though election officials and advocates warn it would still disenfranchise many eligible voters who do not have ready access to these documents.

The executive order makes a number of other sweeping changes, including prohibiting certain election equipment that uses QR codes. Those changes would force states to buy and install new election equipment at significant cost. The order also requires that all ballots must be received by Election Day, an attempt to override states that allow mail ballots that are postmarked by Election Day to be received afterward.

It also asks the Department of Government Efficiency and federal agencies to hunt through state voter rolls in pursuit of ineligible voters, which Levitt said has not been possible for Republicans seeking out voter fraud in the past.