A requirement that voters show photo identification at the polls became law in Pennsylvania on Wednesday, the latest in a spate of Republican-led efforts to impose stricter controls at the ballot box.
Pennsylvania Governor Tom Corbett signed the act into law, saying it set a "simple and clear standard to protect the integrity of our elections."
"I am signing this bill because it protects a sacred principle, one shared by every citizen of this nation," the Republican governor said. "That principle is one person, one vote."
Opponents, who say the measure seeks to suppress voter turnout, vowed to challenge it in court.
Pennsylvania joined several Republican-governed states, including Texas, Kansas and Wisconsin, that have adopted stricter voter identification laws, arguing they were needed to prevent ballot box fraud.
Supporters say the laws are no different from needing identification to board an airplane or obtain a library card.
But some civil rights groups say such laws discriminate against the poor who may not be able to pay fees for copies of legal documents such as birth certificates, and that they could suppress minority votes.
Democrats say voter identification measures are aimed at squeezing out university students and senior citizens who tend to vote for Democrats.
Andy Hoover, legislative director for the American Civil Liberties Union's Pennsylvania branch, said there was no evidence of voter impersonation fraud in the state.
Other states have encountered setbacks trying to impose voter identification requirements. A judge issued an injunction earlier this week against Wisconsin's law, and the U.S. Justice Department blocked a new voter identification law in Texas.
The Justice Department, which also blocked a voter identification law in South Carolina from taking effect, said the Texas law could harm Hispanic voters who lack identification documents.
"Our legal team is currently mapping a strategy for overturning this voter suppression bill," ACLU of Pennsylvania Executive Director Reggie Shuford said in a statement. "The evidence is clear. Some citizens will lose the vote if this becomes law."
Corbett said 99 percent of Pennsylvanians already had valid identification, and those who did not could get one for free at state Department of Transportation driver license centers.
Under the Pennsylvania law, photo identification will not be required for the state's April 24 primary, but will be required for the Nov. 6 general election.
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