An Arizona law requiring proof of citizenship when registering to vote has been struck down by the Supreme Court.
Proponents of the law claimed it reduced voter fraud. Opponents said it was a transparent attempt to discourage voting by legal immigrants and the court, by a rather shocking 7-2 margin, agreed.
Federal law requires you to swear or affirm under penalty of perjury that you are an American citizen and, apparently, the states cannot go significantly beyond that.
The Arizona law would have required naturalized citizens to have registered in person with their actual naturalization document since federal law prohibits copying it.
I'm not too worried about this one because "voter fraud" is a Conservative boogey man. There is absolutely no evidence that any significant fraud actually exists.
Arizona, and other states, still have the option of asking the federal government to add state specific instructions and if refused, can sue the federal government to take action.