The American Civil Liberties Union is challenging a Kansas law requiring proof of citizenship when people register to vote at state motor vehicle offices.

A federal lawsuit filed Thursday contends the documents requirement violates the National Voter Registration Act, a federal law that aims to increase voter participation by eligible citizens.

That law is sometimes called the “Motor-Voter Law” because of a provision requiring states to provide voter registration services in conjunction with drivers’ license applications.

The ACLU lawsuit contends more than 35,000 Kansans have been blocked from voting since Kansas implemented the documentary proof-of-citizenship requirement.

Kansas Secretary of State Kris Kobach did not immediately return phone and email messages.

Dale Ho, the ACLU’s director of voting rights project, contends Kansas has become the nation’s “epicenter of voter suppression.”