A voter shows her ID in a primary election in Cornwall, Pa., on April 24, 2012. (Jeremy Long/AP)

A federal judge in Wisconsin has struck down that state's Voter ID law.

District Judge Lynn Adelman ruled that the state didn't make a compelling case that voter fraud exists and needs to be combated. He also ruled that it's not clear that it would be easy for people without valid photo ID to obtain it and said the law unduly targets minority voters.

"The evidence at trial established that virtually no voter impersonation occurs in Wisconsin," Adelman wrote. "The defendants could not point to a single instance of known voter impersonation occurring in Wisconsin at any time in the recent past."

The decision comes a week after an Arkansas judge struck down that state's new Voter ID law. The Justice Department is also challenging such laws in North Carolina and Texas.

The rulings in Arkansas and Wisconsin are expected to be appealed. Wisconsin Gov. Scott Walker (R) said his state is reviewing its options.

Democrats and the American Civil Liberties Union, which was a plaintiff in the case, hailed the ruling.

"This is a warning to other states that are trying to make it harder for citizens to vote," said Dale Ho, director of the ACLU’s Voting Rights Project. "This decision put them on notice that they can't tamper with citizens' fundamental right to cast a ballot. The people, and our democracy, deserve and demand better."

Such laws passed in many GOP states with new Republican majorities after the 2010 election. Democrats contend that they are aimed at reducing the vote of minorities and other groups that tend to favor their party.

The Wisconsin law was passed before the 2012 election but was not in effect for the November general election after another judge declared it unconstitutional.

Updated at 4:25 p.m.