With the election just 54 days away, the justices did not say when they will decide, although lawyers in the case expected them to rule before the end of September.
The high court appeal follows a lower court's refusal last month to halt the law from taking effect. The 6-month-old law - championed by Republicans over the objections of Democrats - is now part of the heated election-year political rhetoric in a state whose 20 electoral votes make it a major player in electing a president.
The rules - now among the nation's toughest - were already a lightning rod for supporters of President Barack Obama, a Democrat, when a top state Republican lawmaker said in June that the ID requirement "is going to allow" the GOP's presidential nominee, Mitt Romney, to win the state of Pennsylvania. The law has inspired protests, warnings of Election Day chaos and voter education drives.
The six justices - three Republicans and three Democrats - saved their most aggressive questions for lawyers representing the state and Gov. Tom Corbett, a Republican who signed the law in March. A couple exchanges became testy during 80 minutes of arguments.
Justice Thomas Saylor, a Republican, questioned the state's lawyers about whether the law actually requires the state to ensure that every registered voter be able to vote, even those who cannot get a valid ID. Justice Debra Todd, a Democrat, flatly suggested the law is unconstitutional.
Justice Seamus McCaffrey, also a Democrat, pushed the state's lawyers to explain the Republican rationale used to pass the law and whether the Legislature deserves deference for its decision to pass a politically divisive law that "is now going to trample the rights of our citizens."
Republicans have long suspected ballot-box stuffing in the Democratic bastion of Philadelphia, and they contend the photo ID requirement will solidify public confidence in elections. But Democrats say it is a ruse to suppress the votes of minorities, the poor, the young and others considered more likely to vote for Obama.
In the opening statement by a lawyer for the plaintiffs, justices asked whether it would be acceptable for the photo identification requirement to be phased in over a longer period of time - say, a period covering two federal elections. The lawyer, David Gersch, replied that it would, as long as the law guarantees the right to vote to each registered voter, even someone who cannot get a photo ID that is among several types that are valid under the law. Other states, such as Georgia and Michigan, have made such guarantees in their laws, Gersch said.
But under Pennsylvania's law, "there's too little time, there's too many people affected and there's no place in the statute that guarantees that qualified electors can get the ID they need to vote," Gersch told the justices.
Lawyers for the state argued that the justices should defer to the Legislature's decision on a policy matter and to the lower court judge's decision not to halt the law.
The justices quickly put them on the defensive.
"This court is reviewing a refusal to grant a preliminary injunction. Listening to what I'm listening to today, it's as if for some reason we're going to give up the entire history of what we do with preliminary injunctions," a lawyer for Corbett, Alfred Putman, told justices. "You are an appellate court."
The high court normally has seven members. But it heard the case with just six, and a 3-3 deadlock would allow the lower court decision to stand. A seventh justice, a Republican, was suspended in May after being charged in a political corruption investigation.
The plaintiffs include eight individuals, 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. NAACP President Benjamin Todd Jealous was on hand for the arguments.
Several dozen opponents of the law filled the hallway outside the Philadelphia City Hall courtroom after the arguments, many of them minorities or senior citizens.
One, Mary Allen, a retired teacher from Philadelphia, questioned why the law had to be rushed into effect this year, rather than give people more time to get photo IDs.
"It's a move against Obama and Democratic voters," she said.
Levy reported from Harrisburg, Pa.