A federal judge on Wednesday ordered the US Election Assistance Commission to help Kansas and Arizona enforce laws requiring new voters to provide proof of their US citizenship.

US district judge Eric Melgren in Wichita, Kansas, ruled the commission has no legal authority to deny requests from Kansas and Arizona to add state-specific instructions to a national voter registration form. The states and their top election officials — secretaries of state Kris Kobach of Kansas and Ken Bennett of Arizona — sued the agency to force the action.

Both states require new voters to provide proof of their US citizenship to election officials. The federal registration form requires only that prospective voters sign a statement that they are citizens.

While most voters in both states register with state forms, their officials said the availability of the federal form created a loophole as they tried to enforce the proof-of-citizenship requirements.

“I am absolutely delighted,” Kobach said in an interview. “This is a really big victory.”

The federal agency did not immediately return a telephone message seeking comment.

Melgren said the US constitution gives states the power to set voter qualifications, and Congress has not pre-empted it, even in enacting a federal voter-registration law in the 1990s.

“Because the court finds that Congress has not pre-empted state laws requiring proof of citizenship through the National Voter Registration Act, the court finds the decision of the EAC denying the states’ requests to be unlawful and in excess of its statutory authority” Melgren wrote.