A federal judge in Milwaukee struck down Wisconsin's voter identification law Tuesday, declaring that a requirement that voters show a state-issued photo ID at the polls imposes an unfair burden on poor and minority voters.

U.S. District Judge Lynn Adelman sided with opponents of the law, who argued that low-income and minority voters aren't as likely to have photo IDs or the documents needed to get them. Adelman said the law violated the U.S. Constitution's guarantee of equal protection.

The decision invalidates Wisconsin's law and means voter ID likely won't be in place for the fall elections, when Republican Gov. Scott Walker faces re-election. Walker said last month that he would call lawmakers into special session if the courts ruled against the law; Walker's spokeswoman did not immediately return an email seeking reaction Tuesday. Nor did Republican legislative leaders, who have agreed a special session would be warranted to pass a revised version of the law.

Tuesday's decision could set a precedent for similar legal challenges in Texas, North Carolina and elsewhere. There are 31 states with laws in effect requiring voters to show some form of identification, according to the National Conference of State Legislatures. Seven states have strict photo ID requirements similar to the one a state judge struck down in Arkansas last week; that decision has been appealed to the Arkansas Supreme Court. Pennsylvania's voter ID law has been put on hold because of court challenges.

Earlier this month, President Barack Obama waded into the voter ID debate, accusing Republicans of using restrictions to keep voters from the polls and jeopardizing 50 years of expanded voting access for millions of black Americans and other minorities.

A Dane County judge had already blocked Wisconsin's law in state court. The state Supreme Court heard arguments in two separate lawsuits in February, although it's not clear when the justices will issue a ruling. For voter ID to be reinstated, the state's high court would have to rule that it doesn't violate the state constitution, and Adelman's decision would have to be overturned on appeal.

Wisconsin's Department of Justice, which defended the state law in court, pledged to continue the fight.

"I am disappointed with the order and continue to believe Wisconsin's law is constitutional," Attorney General J.B. Van Hollen said in a statement. "We will appeal."

Republican backers had argued that requiring voters to show ID would cut down on voter fraud and boost public confidence in the integrity of the election process. But Adelman said the state failed to prove that voter fraud is a legitimate problem.

"(V)irtually no voter impersonation occurs in Wisconsin and it is exceedingly unlikely that voter impersonation will become a problem in Wisconsin in the foreseeable future," he wrote in a 90-page opinion.

Wisconsin's Republican-led Legislature passed the photo ID requirement in 2011, scoring a long-sought GOP priority. Former Gov. Jim Doyle, a Democrat, had vetoed a similar requirement three times between 2002 and 2005.

Wisconsin's law was only in effect for a 2012 primary before a Dane County judge declared it unconstitutional.

Wisconsin residents can get a free state ID from a Department of Motor Vehicles by presenting documents such as a certified birth certificate, passport or Social Security card. Each document must be unexpired, and the person's name must be spelled identically on each document.

A number of witnesses testified the regulation was a problem, either because their names were misspelled on a key document or because they were born in rural areas during an era when birth certificates weren't always issued.

Adelman cited their testimony in his ruling, noting that they faced challenges that could deter them from voting.

"Although not every voter will face all of these obstacles, many voters will face some of them, particularly those who are low-income," the judge wrote.

The federal challenge combined two separate cases. One was brought by minority-rights groups, including the Wisconsin chapter of the League of United Latin American Citizens, and the other involved the American Civil Liberties Union and the Washington, D.C.-based Advancement Project.

ACLU spokesman Dale Ho said his group was "ecstatic" over the victory, and felt Adelman rendered a fair assessment of the evidence.

"We're pleased. We feel vindicated by the judge's decision," he said.