A U.S. appeals court meanwhile has upheld part of a controversial Arizona ballot measure saying the state can require voters to show identification at the polls. But the court also ruled the state cannot require proof of U.S. citizenship from those registering to vote in federal elections. The ruling adds to a growing debate over voter identification laws enacted in 30 states that critics say disenfranchise people of color, students and the elderly. Last month, the U.S. Justice Department blocked Texas from enforcing a law requiring voters to present photo ID, saying it would discriminate against Latino voters. A similar law has been blocked in Wisconsin.