Nearly 15,000 Kansas residents who registered to vote are having their voting rights held up under a new law requiring voters to prove U.S. citizenship.

The law took effect in January and aims to keep immigrants in the U.S. illegally and other noncitizens from casting fraudulent ballots.

Critics say its enforcement in Kansas and a handful of other Republican-dominated states disenfranchises voters who haven’t produced a birth certificate, passport or other acceptable proof. The voters in limbo in Kansas are enough to swing a close statewide race.

The American Civil Liberties Union is threatening a federal lawsuit.

Secretary of State Kris Kobach promises Kansas will keep enforcing the law. He says the state is lenient in allowing people to fill out registration forms but present papers later.