The law could still take full effect next year, although Simpson could also decide to issue a permanent injunction.
"This decision is a big win for voters in Pennsylvania," said Witold J. Walczak of the American Civil Liberties Union of Pennsylvania, which helped challenge the law.
The plaintiffs included the Homeless Advocacy Project, the League of Women Voters of Pennsylvania and the Pennsylvania chapter of the National Association for the Advancement of Colored People.
Corbett said he still believed that his administration would have made it possible for every registered voter who needed a valid photo ID card to get one.
Election law and voting rights scholars say voter ID requirements stop some people from voting, although it's very hard to determine how many.
"The thing I'm concerned about is that it will lead to confusion on Election Day," said Nathaniel Persily, who teaches election law at Columbia University in New York. "There will be spotty enforcement ... and there could be lines and slow voting as a result."
Michael J. Pitts, who teaches election law at Indiana University, said Pennsylvania's decision is distinctive because of the court's discomfort with changing voter identification requirements so close to an election.
The law was a signature accomplishment of Corbett and Pennsylvania's Republican-controlled Legislature, which passed it over the objection of every Democratic lawmaker. Republicans, long suspicious of ballot box stuffing in the Democratic bastion of Philadelphia, justified it as a bulwark against any potential election fraud.
Democrats, accusing Republicans of using old-fashioned Jim Crow tactics to steal the White House from Obama, turned their opposition to the law into a valuable tool to motivate volunteers and campaign contributions.
Other opponents of the law, including labor unions, good government groups, the NAACP, AARP and the League of Women Voters, have held voter education drives and protest rallies.
Other courts have issued conflicting rulings on strict photo ID provisions. A federal court panel struck down Texas' law, while a state court blocked Wisconsin's voter ID law from taking effect for now. A federal court is reviewing South Carolina's law.