Last month, federal Judge Lynn Adelman struck down the Wisconsin law. On Monday, state Attorney General J.B. Van Hollen filed an appeal.
Wisconsin's Voter ID law requires people to present a government-issue identification card in order to cast a ballot.
Adelman ruled that the law imposes an unfair burden on poor and minority voters, and he did not find widespread problems with the state's prior voting system. Republican state leaders approved the law in 2011, insisting it would deter voter fraud.
Van Hollen appealed the judge's decision in the U.S. Court of Appeals in Chicago, and he asked Adelman to suspend or stay his ruling while the appeal is underway.
Opponents of Voter ID are also fighting the law in the state court system. A state judge halted enforcement in early 2012, so the law was only in effect for one low-turnout election.