PHOENIX — A federal judge in Kansas on Wednesday ordered federal election authorities to help Kansas and Arizona require proof of citizenship of registering voters, in a decision that could well set a trend for other Republican-dominated states.

Judge Eric F. Melgren of United States District Court in Wichita ruled that the federal Election Assistance Commission had no legal authority to deny requests from Kansas and Arizona to add state-specific instructions to a national voter registration form. The states sued the agency to force the action after it had turned them down.

The Supreme Court ruled last June that Congress holds full power over federal election rules, but indicated that states could require proof of citizenship in state and local elections. Federal rules require prospective voters only to sign a form attesting to their citizenship, a procedure favored by Democrats who want to increase participation of minorities and the poor in elections, but that Republican officials say fosters voter fraud.

In his ruling, Judge Melgren, appointed to the bench by President George W. Bush, characterized the decision by the election commission to deny the states’ requests as “unlawful and in excess of its statutory authority.” He said that Congress had not “pre-empted state laws requiring proof of citizenship through the National Voter Registration Act.”