The latest court battle between Arizona and the federal government is being fought in a Pasadena, Calif., courtroom where an appeals court will hear arguments Tuesday on whether the state can require proof of citizenship to register to vote.
An 11-judge panel of the 9th U.S. Circuit Court of Appeals is reconsidering a three-judge panel's ruling that the state's proof of citizenship requirement conflicts with federal voter registration law that law allows people registering to vote to swear under penalty of perjury that they are citizens.

Arizona's law goes further, requiring that people registering show proof of citizenship. It was part of a ballot measure approved by voters in 2004.

The three-judge panel's ruling in October didn't disturb a separate provision requiring voters to provide proof of identity when they cast ballots.

Civil rights groups that challenged Arizona's law argued that thousands of people have had their federal registration forms rejected because they failed to provide documents required by the state.

The U.S. Justice Department 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.