The U.S. Supreme Court has agreed to hear an appeal by the state of Arizona over its voter registration requirements. In 2004, Arizona voters passed a ballot proposition requiring prospective voters to prove they're a citizen before they can register to vote using a state form.

The federal registration form allows voters to swear under penalty of perjury that they're a citizen, but doesn't require proof. However, Arizona had been rejecting any form -- state or federal -- that wasn't accompanied by proof of citizenship.

In August, a U.S. District judge ruled that Arizona cannot reject federal forms just because the state requires proof of citizenship, and said Arizona must make both state and federal registration forms available.

Arizona appealed to the U.S. Supreme Court, and Monday the justices agreed to take up the case.