That means another round of legal briefs and courtroom arguments. At the same time, advocates say other groups have been eyeing a parallel challenge in federal court.
Simpson's order denied a bid to block the state from implementing the law, which requires voters to show acceptable identification before casting ballots.
Opponents complained that the law could unfairly keep Pennsylvanians from the polls, particularly people who do not have or could not get such IDs in time. Supporters say the law was necessary to prevent election fraud, and disputed that its impact would be so far-reaching.
Nineteen states have toughened their voter identification laws in the last two years. Challenges are pending in more than a third, including Wisconsin, South Carolina, Texas, and Florida.
The Pennsylvania case ranks among the most watched because of its potential to affect the presidential race in a critical swing state. That's also a reason observers expect the state's high court to put the case on a fast track.
"Everybody involved here understands the stakes," said Seth Kreimer, a constitutional law professor at the University of Pennsylvania and consultant to the American Civil Liberties Union, one of the petitioners in the case.
A quirk of timing on the state Supreme Court could add another subplot.
With Justice Joan Orie Melvin suspended, the court has just six members, an even split of Democrats and Republicans. Party affiliation might be deemed irrelevant in many cases they hear, but politics are baked deeply into this one.
Republicans pushed the law through the legislature with a party-line vote, and House Majority Leader Mike Turzai (R., Allegheny) famously declared that the law would help GOP presidential nominee Mitt Romney win the state. Democrats pounced on that, contending it proved the law was concocted for political reasons.
If the high court splits 3-3 on the issue, Simpson's ruling would remain intact. For the Supreme Court to settle the case, one justice would have to buck his or her party.
Observers are particularly focused on the role Chief Justice Ronald Castille, a Republican, could play.
Castille irked some Republicans when he delivered the majority opinion this year that threw out a plan by GOP lawmakers to redraw Pennsylvania's legislative map. His role in the voter ID case could burnish his party bona fides or burn them.
"We already had one political test case, and he was the swing vote," said G. Terry Madonna, who directs the Center for Politics and Public Affairs at Franklin and Marshall College.
Despite the law's national ramifications, the state courts should be the final arbiter.
"Once the state constitution has been interpreted by the state courts, the federal courts have, one would think, no role to play," Kreimer said. But he cautioned that nothing was guaranteed.
"People can presumably get creative," he said, referring to the legal battle that led Florida's disputed 2000 presidential election results to the U.S. Supreme Court.
What's more likely are federal lawsuits challenging the law as a violation of the Voting Rights Act.
During the trial before Simpson, opponents presented survey results arguing that the law disproportionately impacted Latino voters, who were less likely to have valid state-issued identification needed to cast ballots. Advocates could take a similar tack to federal court, presenting the case as a civil rights matter.
"I would frankly be surprised if there are not [federal] lawsuits filed," Witold "Vic" Walczak, state legal director of the ACLU, said before Simpson's decision came down.
State officials are standing pat. The Department of State spent months training local election officials on the changes required by the law, spokesman Ron Ruman said, and had no contingency plans.
"We don't necessarily have a Plan A or Plan B," Ruman said last week, "because there are too many possibilities."
Contact John P. Martin at 215-854-4774, firstname.lastname@example.org, or follow @JPMartinInky on Twitter.