Simpson's ruling could be appealed to the state Supreme Court, although state officials weren't ready to say Tuesday whether they would appeal. He based his decision on guidelines given to him days ago by the high court justices, and it could easily be the final word on the law just five weeks before the Nov. 6 election.
Simpson's ruling will allow the law to go into full effect next year, though he could still decide later to issue a permanent injunction.
Election workers will still be allowed to ask voters for a valid photo ID, but people without it can use a regular voting machine in the polling place and would not have to cast a provisional ballot or prove their identity to election officials afterward.
One lawyer for the plaintiffs called it a "win," while the Advancement Project, which aided the legal challenge, expressed concern that a new public education campaign would be needed to ensure people without photo ID know they can vote.
Gov. Tom Corbett, a Republican who helped champion the law, said the state's lawyers were still analyzing it.
The state's Republican Party chairman, Rob Gleason, said he was disappointed and stressed that the law is a "common-sense reform" that is supported in public polling across the political spectrum.
"Despite the empty rhetoric to the contrary, this legislation is still about ensuring one person, one vote," Gleason said.
In a statement, the Obama campaign said the decision means that "eligible voters can vote on Election Day, just like they have in previous elections in the state. "
Simpson's ruling came after listening to two days of testimony about the state's eleventh-hour efforts to make it easier to get a valid photo ID. He also heard about long lines and ill-informed clerks at driver's license centers and identification requirements that made it hard for some registered voters to get a state-issued photo ID.
The 6-month-old law now among the nation's toughest has sparked a divisive debate over voting rights and become a high-profile political issue in the contest between Obama, a Democrat, and Republican nominee Mitt Romney, for Pennsylvania's 20 electoral votes.
It was already a political lightning rod when a top state Republican lawmaker boasted to a GOP dinner in June that the ID requirement "is going to allow Gov. Romney to win the state of Pennsylvania."
Pennsylvania, traditionally considered one of the most valuable presidential swing states, is showing a persistent lead for Obama in independent polls. Pollsters had said Pennsylvania's identification requirement could mean that fewer people ended up voting and, in the past, lower turnouts have benefited Republicans in Pennsylvania.
But Democrats have used their opposition to the law as a rallying cry, turning it into a valuable tool to motivate volunteers and campaign contributions while other opponents of the law, including labor unions, good government groups, the NAACP, AARP and the League of Women Voters, hold voter education drives and protest rallies.
The law was a signature accomplishment of Pennsylvania's Republican-controlled Legislature and Corbett. Republicans, long suspicious of ballot-box stuffing in the Democratic bastion of Philadelphia, justified it as a bulwark against any potential election fraud.
Every Democratic lawmaker voted against it. Some accused Republicans of using old-fashioned Jim Crow tactics to steal the White House from Obama. Other opponents said it would make it harder for young adults, minorities, the elderly, poor and disabled to vote.
A wave of new voter identification requirements have been approved in the past couple years, primarily by Republican-controlled Legislatures.
Earlier this year, a federal court panel struck down Texas' voter ID law, and a state court in Wisconsin has blocked its voter ID laws for now. The Justice Department cleared New Hampshire's voter ID law, and a federal court is reviewing South Carolina's law. Where other states stand
In several states, the status of voter ID rules remain unresolved. Here's where they stand.