A federal appeals court ruled Monday that Wisconsin's requirement that voters show photo identification at the polls is constitutional, a decision that is not surprising after the court last month allowed for the law to be implemented while it considered the case.

State elections officials are preparing for the photo ID law to be in effect for the Nov. 4 election, even as opponents continue their legal fight. The American Civil Liberties Union and the Advancement Project asked the U.S. Supreme Court last week to take emergency action and block the law.

Opponents argue that requiring voters to show photo ID, a requirement that had, until recently, been on hold since a low-turnout February 2012 primary, will create chaos and confusion at the polls. But supporters say most people already have a valid ID and, if they don't, there is time to get one before the election.

The opinion from the 7th U.S. Circuit Court of Appeals comes a month before the election involving the closely watched race between Republican Gov. Scott Walker, who supports the law, and Democratic challenger Mary Burke.

A lower court judge, U.S. District Judge Lynn Adelman, struck the law down as unconstitutional in April, saying it unfairly burdens poor and minority voters who may lack such identification. Republican Attorney General J.B. Van Hollen asked the 7th Circuit to overturn that ruling.

The three-judge panel agreed with Van Hollen. The judges said Wisconsin's law is substantially similar to one in Indiana that the U.S Supreme Court declared was constitutional.

A spokeswoman for Van Hollen did not immediately return a message seeking comment.

Dale Ho, director of the ACLU's Voting Rights Project, said in response that "the voters of Wisconsin deserve every opportunity to cast their ballot free of the obstacles imposed by this law, and we are evaluating our next step."

The Republican-controlled Legislature passed the law and Walker signed it in 2011, saying it was an important safeguard against voter fraud. But opponents argued claims of fraud were overblown and the real intent was to disenfranchise Democratic-leaning constituencies -- including minorities and poor people -- from voting.

Because of legal challenges, the law had been on hold since the February 2012 primary. But since the Sept. 12 ruling allowing the law to go into effect pending the court's final decision, Wisconsin elections officials have been working to educate voters about the requirement.

The Wisconsin Government Accountability Board, which oversees elections, continues to work with local officials to implement the law "as we have since the court's September 12 order," said GAB spokesman Reid Magney in reaction to the court's ruling.

Wisconsin's law requires people to show certain government-issued photo ID at the polls to vote. Anyone who doesn't have the proper ID on Election Day can cast a provisional ballot, and they would then have until 4 p.m. on the Friday after the election to present the required ID to have the vote counted.

Judge Frank Easterbrook, a Ronald Reagan appointee, wrote the opinion and was joined by judges Diane Sykes and John Tinder, both appointees of Republican President George W. Bush.