The system, which requires proof of citizenship to vote in state and local elections, is seen by some activist groups as an attempt to discourage voters who might favor Democratic candidates, The New York Times reported Saturday.
The states devised the two-tier system after the U.S. Supreme Court in June struck down citizenship requirements for federal elections but did not extend the ban to state elections.
"If you require evidence of citizenship, it helps prevent people who are not citizens from voting, and I simply don't see a problem with that," said Arizona Attorney General Tom Horne.
An initial comparison this week of Maricopa County voters who registered using a federal form and those who used a state form that required proof of citizenship turned up 900 people, out of 1.9 million registered voters, who did not show proof of citizenship.
Opponents -- and election officials -- say the system will be expensive and difficult to execute, and will create confusion at the ballot box.
Petra Falcon, executive director of Promise Arizona in Action, a group promoting voter-registration efforts in Phoenix, termed the system "another veiled attempt at discouraging young voters, low-income voters, Latino voters from entering the electoral process."
Illinois and Mississippi have tried the dual-registration system and their efforts didn't last. Both were rejected in state and federal courts.