WASHINGTON – Supreme Court justices disagreed Monday over whether states can require would-be voters to prove they are U.S. citizens before using a federal registration system designed to make signing up easier.

Arizona and other states told the justices the precaution is needed to keep illegal immigrants and other noncitizens from voting. But some justices asked whether states have the right to force people to document their citizenship when Congress ordered the states to accept and use federal “motor voter” registration cards that only ask registrants to swear on paper that they are U.S. citizens.

“I have a real big disconnect with how you can be saying you’re accepting and using, when you’re not registering people when they use it the way the federal law permits them to,” Justice Sonia Sotomayor said to Arizona Attorney General Thomas C. Horne.