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.