The citizenship requirement stems from Proposition 200, approved by Arizona voters in 2004. The law also denied some government benefits to illegal immigrants and required Arizonans to show identification before voting. The 9th Circuit upheld the voter identification provision. The denial of benefits was not challenged.
Soon after voter's approved the law, Latino, native American and other rights advocacy groups filed lawsuits challenging the voter registration provision and other aspects of the law.
The appeals court has issued multiple rulings in the case concerning the need for registrants to prove they are U.S. citizens. A three-judge panel initially sided with Arizona. A second panel that included retired Supreme Court Justice Sandra Day O'Connor, who from time to time sits on appeals courts, reversed course and blocked the registration requirement.
At that point, the appeals court judges voted to have the case heard by the larger, en banc court. The federal and state governments share responsibility for elections and this dispute is over where the line should be drawn between them.