The 4-2 decision by the state Supreme Court sends the case back to a Commonwealth Court judge who initially said the divisive law could go forward. The high court asked the judge, Robert Simpson, for his opinion by Oct. 2.
If Simpson finds there will be no voter disenfranchisement and that IDs are easily obtained, then the 6-month-old law can stand, the Supreme Court said.
"It's certainly a very positive step in the right direction in that the court recognizes that the state does not make adequate provision for people to get the ID that they would need to vote," said David Gersch, the lead lawyer for the plaintiffs challenging the law's constitutionality. "In addition, there is a practical problem with getting the ID to people in the short time available."
A spokesman for the Pennsylvania Department of State, which oversees voting and elections, said the agency will provide whatever information that Simpson may seek.
"We believe, as we have all along, that any legal voter who wants to get an ID is able to do so," spokesman Ron Ruman said.
The court's three sitting Republican justices were joined in the majority by one of the court's Democrats, Max Baer. The court's two other Democrats dissented.
Part of the problem, the four majority justices noted, is that the state has scrambled to deal with impediments to distributing a photo ID card promised under the law to any registered voter who needs one. The state began issuing new, voting-only ID cards in late August, after Simpson's original ruling.
"Thus, we will return the matter to the Commonwealth Court to make a present assessment of the actual availability of the alternate identification cards on a developed record in light of the experience since the time the cards became available," the justices wrote.
The justices also pointed to Gersch's suggestion during last week's oral arguments that the photo ID requirement can be constitutional if the state has ensured all eligible voters can get the photo IDs they need.
Some of the people who sued over the law had raised the claim that they might be unable to vote because they lacked the necessary documents, such as an official birth record, to get the law's ID card of last resort: A state nondriver photo ID that is subject to strict federal requirements.
In oral arguments Sept. 13 before the Supreme Court, Justice Thomas Saylor pointed out that the state cannot comply with the "letter of the law," which calls for issuing an ID card to registered voters who need it, because the cards still require supplemental identification that a registered voter might not be able to produce.
The Republican-penned ID law passed over the objections of Democrats and ignited a furious debate over voting rights, making it a high-profile issue in the contest for the state's prized 20 electoral votes between President Barack Obama, a Democrat, and Republican nominee Mitt Romney, the former Massachusetts governor.
Republicans, long suspicious of ballot-box stuffing in the Democratic bastion of Philadelphia, say the law would deter election fraud. But Democrats pointed to a blank trail of evidence of such fraud, and charged that Republicans are trying to steal the White House by making it harder for the elderly, disabled, minorities, the poor and college students to vote.
The law - among the nation's toughest - has inspired protests, warnings of Election Day chaos and voter education drives. 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."
The plaintiffs - eight registered Democrats, plus 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 - had sought to block the law from taking effect in this year's election as part of a wider challenge to its constitutionality.
Republican Gov. Tom Corbett signed the law in March after every single Democratic lawmaker voted against it.
The prior law required identification only for people voting in a polling place for the first time and it allowed nonphoto documents such as a utility bill or bank statement.