A U.S. appeals court has ordered that Wisconsin’s voter ID law go into effect immediately, raising the prospect of chaos and confusion at the polls this fall.

A three-judge panel made the ruling after hearing an appeal Friday by the state of Wisconsin. The ID law had been struck down by Judge Lynn Adelman in April, who ruled that it violated the Voting Rights Act’s ban on racial discrimination.

“The panel has concluded that the state’s probability of success on the merits of this appeal is sufficiently great that the state should be allowed to implement its law, pending further order of this court,” the judges wrote.

The ruling noted that since the April ruling striking down the law, the procedures had been revised to make it easier to get ID, lessening the burden on voters.

All three of the judges on the appeals court panel were Republican appointees.

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The law, passed in 2012, has never gone into effect thanks to legal challenges. Instating it so close to the November election—early voting starts next month—could be challenging for the state’s election administrators.

Rick Hasen, a prominent election law expert, called the ruling “a big big mistake for election administrations.”

“Making changes in election rules as voting gets underway (think of overseas and military voters, for whom the process starts 45 days before election) is likely to create a great deal of confusion and uncertainty,” Hasen wrote online. “It is hard enough to administer an election with set rules—much less to change the rules midstream.”

Hasen said the plaintiffs could make an emergency motion to the Supreme Court, and that there’s a “decent chance” the Justices might intervene.

“This decision fails to protect the voters of Wisconsin and the integrity of our elections,” Dale Ho, director of the ACLU’s Voting Rights Project, said in a statement released late Friday. “Before today, courts considering ID laws have not allowed them to go into effect this close to an election. We are evaluating our options to prevent chaos at the polls and to ensure that all Wisconsin voters are able to access the ballot free of unjustified obstacles.”

In one sign of the potential confusion, a spokesman for the Government Accountability Board, which runs the state’s elections, said earlier Friday that 11,815 absentee ballots had already been mailed to voters without photo ID instructions.

If Adelman’s ruling is overturned, as now looks likely, it would deprive voting rights advocates of a valuable precedent that they’d hoped to use to help strike down other voter ID laws in Texas and North Carolina via the Voting Rights Act.

Gov. Scott Walker, who signed the ID law and has strongly defended it, will be on the ballot this fall, in what could be a tight race.