In 2008, the U.S. Supreme Court in a widely cited opinion said states could require voters to show photo identification at the polls to guard against fraud.
But that decision was not the last word on voter ID.
In Wisconsin, two judges recently issued injunctions against that state's voter ID law, saying it presented real hurdles to casting a ballot.
Missouri's state Supreme Court struck down the photo ID requirement there. Now the state has a weaker ID law that allows voters to submit utility bills, bank statements, and other documents as identification, without a photo.
Court outcomes have swung so widely that legal experts are wary of predicting how Pennsylvania Commonwealth Court Judge Robert E. Simpson will rule later this month on a challenge to the state's new voter ID law. Simpson heard seven days of testimony that wrapped up Thursday.
"There are both factual and legal differences" from laws and court challenges in other states, said Richard L. Hasen, a professor at Loyola Law School in Los Angeles and author of The Voting Wars, which chronicles the fight over election laws starting with the battle between George W. Bush and Al Gore in 2000.
The Pennsylvania case has generated suspense, he added.
"Pennsylvania is the only potential swing state that I'm aware of where the question of whether voter ID laws will be in place is uncertain this close to the election," he said.
Indiana was among the first to adopt a voter ID law, in 2006. The Indianapolis League of Women Voters sued in federal and state courts, contending the law disenfranchised voters.
The league eventually lost both cases. When the U.S. Supreme Court upheld the Indiana law in 2008, Justice John Paul Stevens said preventing fraud was a legitimate concern.
"The application of the statute to the vast majority of Indiana voters is amply justified by the valid interest in protecting the integrity and reliability of the electoral process," he wrote in the majority opinion.
But the Supreme Court ruling may not apply in every case. The justices said the plaintiffs had failed to present evidence that the law violated anyone's constitutional rights, and said such proof might make future challenges possible.
Robert Pastor, director of American University's Center for Democracy and Election Management, said the Pennsylvania lawsuit seemed to present the best evidence of harm of any case so far. The American Civil Liberties Union of Pennsylvania brought the suit and has presented testimony from 10 people, some of them elderly, who said it would be difficult for them to get the required ID before the November election.
"Up until this case, opponents of voter ID made judgments about likely effects without having empirical evidence," he said.
Indiana's law fed a wave of similar legislation in state legislatures, often promoted by the American Legislative Council (ALEC), a controversial group funded by conservative and corporate donors.
As a result, 33 states have some form of voter ID law, according to the National Conference of State Legislatures.
The U.S. Department of Justice is fighting those laws in court in Texas and South Carolina, saying they violate the federal Voting Rights Act.
The landmark 1965 act aimed to end literacy tests and other requirements that prevented people, many of them African American, from voting. Opponents of the new voter ID laws argue that requiring identification presents a significant barrier to casting a ballot. Supporters say it is no different from current laws that require people to register to vote.
Some conservatives have attacked U.S. Attorney General Eric Holder over the federal actions, arguing that he is just trying to help President Obama win a second term.
Last month, the Justice Department opened an investigation into Pennsylvania's law, though it has not determined whether it will file suit.
The Voting Rights Act works differently in Pennsylvania than in Texas and South Carolina. That is because Texas and South Carolina belong to a group of states, most of them in the South, that have a history of voting discrimination.
Those states can't change their voting laws without getting approval -- officially known as "preclearance" -- from the Justice Department. After Texas and South Carolina passed voter ID laws in 2011, the Justice Department refused to grant preclearance, arguing that the new requirements would be discriminatory.
Pennsylvania does not need preclearance from the Justice Department, which so far has only requested databases and other information related to the new ID law.
If the Justice Department were to bring legal action against Pennsylvania over the issue, it would have to prove that the law here discriminates. That's a bigger burden than in preclearance states, which must prove that the change will not disenfranchise certain groups.
"The question is, who has the advantage?" said Michael J. Pitts, a law professor at Indiana University who worked on Voting Rights Act cases in the Justice Department from 2001 to 2005.
In preclearance states, the Justice Department has the edge. In states such as Pennsylvania, the state does.
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