Arizona's voter registration law was under the microscope Monday at the U.S. Supreme Court.
The justices are being asked to decide whether the state went too far by requiring voters to provide proof of citizenship.
Arizonans approved the law back in 2004 as part of a broader effort to crack down on illegal immigrants.
Opponents call it an undue burden on legal residents.
Native Americans, for instance, may not have birth certificates, but they are citizens.
"The law has a terrible impact on the voting population and the majority of people who were unable to vote were white," notes the ACLU's Laura Murphy.
The 1993 Voter Registration Act says voters can simply sign an affidavit.
"Congress says a simple post card. This certainly leaves the states free to do whatever checking they want to do on their own," notes Latino civil rights advocate Nina Perales.
But Arizona wants more than a promise.
The state wants proof of citizenship.
"I think that there are many indications Congress did not intend to prevent us from asking for evidence of citizenship," argues Arizona Attorney General Tom Horne.
Alabama, Georgia, Kansas and Tennessee require similar documentation.
A dozen other states are considering their own laws.
Justice Anthony Kennedy, the key swing vote on this court, expressed skepticism about the law, which complicates a voting process that congress was trying to simplify.
--Brian Mooar, NBC News reports