WASHINGTON — The Supreme Court argued Monday over whether states fighting voter fraud and illegal immigration can make people document their U.S. citizenship before allowing them to use a federal voter-registration system designed to make it easier to vote.

Several justices questioned whether Arizona and other states have the right to force people to document their citizenships when Congress didn’t require it in ordering states to accept and use the “Motor Voter” registration card. But other justices said states should be able to police the citizenship of voters since the federal government only asks people to swear on paper that they’re U.S. citizens. “This is proof? This is not proof at all,” said Justice Antonin Scalia, who sounded skeptical of the opponents of Arizona’s law.

But lawyer Patricia Millett, representing those challenging the law, answered that courts accept sworn statements as proof in procedures “by which we execute people.”

Congress decided that a sworn statement with the risk of perjury was sufficient to register to vote in the federal system, she said, and not “a ticket into the state’s own registration system” where states can ask for additional information.

The court is deciding the legality of Arizona’s requirement that prospective voters document their U.S. citizenship in order to use a registration form produced under a federal voter registration law. The 9th U.S. Circuit Court of Appeals said the 1993 National Voter Registration Act, which doesn’t require that documentation, trumped Arizona’s Proposition 200 passed in 2004.

Arizona appealed that decision to the Supreme Court.

Four other states — Alabama, Georgia, Kansas and Tennessee — have similar requirements, and 12 other states are contemplating similar legislation, officials said.

The federal 1993 “Motor Voter” law allows would-be voters to fill out a mail-in voter registration card and swear they are citizens under penalty of perjury, but it doesn’t require them to show proof. Under Proposition 200, Arizona officials require an Arizona driver’s license issued after 1996, a U.S. birth certificate, a passport or other similar document, or the state will reject the federal registration application form.