Arizona Attorney General Tom Horne wants a federal commission to approve an Arizona requirement that people using a federal form to register to vote prove they are citizens.
The U.S. Supreme Court in June ruled that Arizona could not require such proof from people using the form allowed under the federal "motor voter" law.
But the high court said Arizona could ask the Election Assistance Commission to approve the citizenship proof requirement. If the commission refused, Arizona could ask a federal court to overturn the commission's decision.
Horne released a letter on Wednesday that he wrote to the commission saying he would sue if they don't allow the proof requirement by Aug. 19.
Most Arizonans use a state form that requires citizenship proof required under Proposition 200, which voters approved in 2004.