Secretary of State Kris Kobach says Kansas will continue to enforce a law requiring new voters to provide proof of their U.S. citizenship despite a threatened lawsuit.

Kobach said in a statement Tuesday that he’s not surprised the American Civil Liberties Union is threatening to sue on behalf of other groups, including the NAACP and League of Women Voters. He said liberal groups have made it clear they oppose the rule.

Federal law says groups intending to sue over such issues must give 90 days’ notice. The ACLU did that Tuesday.

Kobach said Kansas will keep taking its requirement “seriously.”

The ACLU contends Kansas is violating a federal election law aimed at ensuring people can register to vote at state driver’s license offices.