In the ruling, McGinley concluded the 2012 law created insurmountable obstacles for hundreds of thousands of people to vote, many of them elderly and disabled.
"Voting laws are designed to assure a free and fair election; the voter ID law does not further this goal," he wrote.
Michael Rubin of the plantiffs' law firm, Arnold & Porter, called the ruling "a devastating indictment of Pennsylvania's voter-ID law."
But Hasen said there were parts of McGinley's ruling - such as his charge that the law did not violate the state's equal protection provision - that "actually work in favor of those who support the constitutionality or legality of voter-ID laws."
The national introduction of voter-ID provisions dates back more than a decade: According to the National Conference of State Legislatures, 24 states passed voter-ID bills between 2003 and 2012.
Pennsylvania's law, signed by Gov. Corbett, a Republican, was considered one of the nation's strictest, and it drew protest from every Democratic lawmaker in the GOP-controlled legislature.
Neither Corbett nor his top lawyer would say Friday whether they would file an appeal of McGinley's ruling, though many observers expect one.
Marian Schneider, an attorney with the Advancement Project, a civil rights group supporting the plaintiffs, was wary of predicting how the justices would rule.
"If there's an appeal, we hope that we'll prevail," she said.
Even then, the ruling might not reverberate beyond the commonwealth.
Unlike challenges in Texas and North Carolina - which face federal civil rights lawsuits from the Justice Department over their voter-ID laws - many of the questions in Pennsylvania's case centered on the state Constitution.
"Every state has their own set of constitutional provisions and statutes," Hasen said.
Montgomery County Commissioner Bruce L. Castor, a Republican, called Friday's decision "extraordinary."
"When a court overturns a law, that's a big deal because that's a repudiation of the democratic process that created the law," he said.
The county distributed about 100 of its own identification cards to residents ahead of the 2012 election.
Castor said he thought the Supreme Court would render a decision that fixes the law or gives instructions on how to fix it.
"Ultimately, I suspect we'll have some kind of identification requirement," he said. "Just one that is satisfactory to our legal system."