PHOENIX (AP) – Federal judges engaged in a lively debate with lawyers Tuesday over Arizona's effort to require people to show proof of citizenship when they register to vote.

Federal voter registration law allows people to submit a mail-in voter registration card and swear they are citizens under penalty of perjury. But Arizona's law – approved by voters in 2004 as part of a ballot measure – seeks further documentation, such as a driver's license.

An 11-judge panel from the 9th U.S. Circuit Court of Appeals lobbed questions at Arizona Attorney General Tom Horne, representatives of minority groups that have challenged the Arizona law and a Department of Justice lawyer from Washington, D.C.

Nina Perales of the Mexican-American Legal Defense and Education Fund said at least 30,000 potential voters have been excluded from voting in Arizona because they failed to provide other documents required by the state – even though there was no evidence they weren't eligible to vote. Not all Arizona residents have driver's licenses – just 90 percent, she said.

Attorney Jon M. Greenbaum, a civil rights lawyer representing Indian tribes, said the added requirements works against those who live on Indian reservations.

“The federal form was supposed to make registration simple,” he said. “Members of Indian tribes are sometimes 90 miles away from a registration office where they could provide documentation.”

Chief Judge Alex Kozinski interjected. “But not 90 miles away from a mailbox, right?” he asked.

Greenbaum replied that anyone on an Indian reservation should be able to mail in the postcard and simplify the process.

“Arizona is undermining that,” he said.

Horne said it wasn't unreasonable for the state to seek more documentation, saying the proof could also be mailed in an envelope. He was added that some non-citizens have been tricked into signing postcards by voter registration organizations that have sent people door to door.

The U.S. Justice Department has filed a friend-of-the-court brief urging the 9th Circuit to overturn the state law, which the brief said is invalid because it conflicts with the National Voter Registration Act. The act, which requires states to “accept and use” the federal form, was intended to simplify and standardize voter registration procedures nationwide, the federal government's lawyers said.

Horne's office said in a brief that the proof of citizenship requirement was not burdensome because the vast majority of prospective voters only have to provide a number from a driver's license, tribal identification card or certificate of naturalization, not produce an actual document.

Arizona's voter registration law had been upheld by state and federal courts until the three-judge panel's October ruling. It overturned a previous 9th Circuit panel's ruling that found Proposition 200 did not violate the National Voter Registration Act.

In the two-judge majority in the October ruling, Judge Sandra Ikuta said the state cannot impose more onerous requirements than those laid out under the National Voter Registration Act. The panel's third judge dissented, saying the ruling ignored precedent and was flat wrong on its legal analysis.

Lawyers on both sides said after Tuesday's hearing that they expected another split decision from the larger panel, and whichever side loses would likely take the matter to the U.S. Supreme Court.