A federal judge struck down Wisconsin's voter identification law Tuesday, saying the measure places an unnecessary burden on poor and minority voters, court documents showed.

The ruling comes amid Supreme Court decisions that question the relevance of civil rights laws put in place during the 1960s to combat discriminatory state statutes on voting.

U.S. District Judge Lynn Adelman wrote in the ruling that the law could have a disproportionate impact on such voters because of the costs some of them might incur in obtaining a photo ID, if they did not already have one, according to online court documents.

The ruling 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.

Minorities in Wisconsin are disproportionately likely to live in poverty and those who live in poverty are less likely to drive or participate in other activities such as banking and traveling, in which a photo ID is required, Adelman wrote in his ruling.

"Thus, we find that Blacks and Latinos are less likely than whites to obtain a photo ID in the ordinary course of their lives and are more likely to be without one," he wrote.

Wisconsin Attorney General J.B. Van Hollen said in a statement that he is "disappointed" with the ruling and plans to appeal.

Al Jazeera and wire services