Kansas Secretary of State Kris Kobach may have blocked more than 30,000 potential voters by failing to rescind a policy requiring residents to show proof of citizenship when they register to vote,  a lawsuit filed Thursday by the American Civil Liberties Union charges.

A federal judge last month told Kobach he had overstepped his authority by asking residents to show documentary proof of citizenship when they register. But despite the judge’s opinion, Kansans who have been trying to register to vote at state Department of Motor Vehicle offices are being asked to provide additional proof of citizenship, the ACLU said. When they can’t produce the documentation, Kobach’s office has suspended voters from participating in state elections — a policy that has affected ordinary citizens and military service members, the ACLU said.

"What's happening in Kansas is outrageous," Dale Ho, director of the ACLU's Voting Rights Project, said in a statement. "These shameful actions have made Kansas an epicenter of voter suppression. We say no more barriers. Let people vote."

The National Voter Registration Act, a federal law meant to streamline registration in the U.S., requires states to allow people to register to vote when they apply for or renew a state government-issued ID or driver’s license. Kobach, a conservative Republican who has been a leading voice in efforts by state officials to crack down on undocumented immigration, sued the federal government in an attempt to force a proof-of-citizenship requirement on federal voter registration forms.

The U.S. Supreme Court eventually ruled against Kobach in 2013, saying that states could not require such documentation on federal forms, the Huffington Post reported. In response, the secretary of state’s office implemented a dual voter registration system, in which voters who did not have citizenship documentation could only vote in federal elections.

But on Jan. 15, a district state judge in Kansas said Kobach could not require state residents to show documents such as passports or birth certificates during the registration process or relegate residents into two classes of voters. “The secretary is not empowered to determine or declare the method of registration or create a method of 'partial registration' only,” Judge Franklin Theis wrote in his ruling, according to the Huffington Post.

“In Kansas, a person is either registered to vote or he or she is not," Theis added.

The ACLU’s lawsuit, Fish v. Kobach, was filed in U.S. District Court in Kansas City, Kansas, and asks the court to force state officials to immediately register thousands who were denied registration at DMV offices over the citizenship documentation requirements.

"Ordinary people who play by the rules and follow all necessary instructions still end up not being able to register to vote because of this bureaucratic maze," said Ho, the ACLU voting rights lawyer.