Critics of a Kansas law requiring new voters to provide proof of their U.S. citizenship when registering want legislators to repeal it during their special session.

The American Civil Liberties Union said Tuesday that lawmakers must act because more than 15,000 legal residents have their voter registrations on hold because they haven’t provided proof of their citizenship. Joining the ACLU were the NAACP and Equality Kansas, the state’s leading gay-rights group.

Those groups already have told Secretary of State Kris Kobach that they might file a federal lawsuit. Kobach contends the law prevents election fraud.

Lawmakers were returning Tuesday to Topeka to repair a law allowing convicted murderers to be sentenced to at least 50 years in prison, and their leaders had no plans to take up other topics.