Three justices who had been elected to the court as Republicans voted to uphold the plan. Three who had been elected as Democrats voted to reject it.
The deciding vote for rejection was cast by Chief Justice Ronald D. Castille, formerly a Republican district attorney in Philadelphia.
It marked the first time since adoption of a revised state constitution in 1968 - which first set up a legislative reapportionment commission - that a commission's plan had been overturned.
"Holy cow," said Tim Potts, founder of the good-government group Democracy Rising. "The forces of truth and justice have another opportunity to prevail. With luck, the commission will look at an alternative plan."
Both Democrats and Republicans were left somewhat at sea because the court did not explain its decision. It said it would spell out its reasoning in an opinion yet to come.
Democrats said the court's move appeared to lock in place, at least until after this year, the district boundaries adopted in 2001. They said they could not see how the commission could go back to the drawing table and do a remapping in time for the April 24 primary.
Republicans, however, said they were not certain that the current boundaries were locked in. They said there could be time for revisions if the court asked for only minor changes.
"I'm uncertain at this point if the plan is remanded for a major overhaul or to correct a minor problem," said Andrew Reilly, Republican chairman of Delaware County.
"It may very well be locked in for this election. But I think it's too premature to say that."
The court decision came just as candidates for House and Senate districts were starting to go from door to door - in what they thought were their districts - to collect voter signatures on nominating petitions for the primary.
A Philadelphia seat being given up by Rep. Dennis M. O'Brien, a Republican, after his election to City Council was to have been moved to York County. It will stay in the city, at least temporarily.
Robert A. Gleason Jr., the state GOP chairman, said his party had expected to capture that seat after the move to largely Republican central Pennsylvania. If it remains in the city, it is likely to go Democratic, he said.
"That could cause some angst," he said of the court's move.
Democrats mostly were flying high after the decision.
"Needless to say, we are very pleased with the ruling of the court," said Senate Minority Leader Jay Costa of Allegheny County, who represented his caucus on the reapportionment panel and voted against its plan.
"We thought there was a better way to draw the House and Senate lines. . . . We await the [court] opinion that will give us instructions and guidance."
J. Manly Parks, solicitor of the Delaware County Democratic Party, who argued against the new plan in court, said the decision means that four predominantly Democratic county boroughs - Swarthmore, Darby, Yeadon, and Collingdale - would not be split. Each will have a single state House representative and not be parceled out among two or more.
Parks and other appellants argued that the constitution requires counties, municipalities, and even Philadelphia wards to be kept together unless it is "absolutely necessary" to divide them.
Supporters of the commission plan argued that while that was true, a higher constitutional requirement is for every district to have nearly the same population.
Cliff Levine, a Pittsburgh lawyer who argued on behalf of Senate Democrats in the case, said the court appeared to buy the Democratic argument.
Levine said both requirements could be met.
"I think this is a pretty historic decision," he said of the plan's rejection.
In a dissenting statement written by Justice Thomas Saylor, three justices said they weren't certain the plan violated court precedent.
Saylor, joined by J. Michael Eakin and Joan Orie Melvin, wrote that "although I am receptive to the concern that past decisions of the court may suggest an unnecessarily stringent approach to equalization of population as between voting districts, I believe this could be addressed via prospective guidance from the court."
Senate Majority Leader Dominic Pileggi, a Delaware County Republican who served on the commission and voted yes on its report, said he was mystified by the ruling.
"We believe what was approved was in full compliance with existing precedent, the state constitution, and federal rules," Pileggi said.
"I think any objective observers would say the commission plan is superior to the 2001 existing plan in terms of number of municipalities split and population metrics."
He said he expected the commission to meet again as early as next week. If the court's instructions for fixes are minor, he said, changes could be made in a matter of days.
But if the required changes are broad, it could take months to redraw the map, he said, adding: "This is new territory."
The commission plan, as submitted to the court, was not without Democratic support. House Minority Leader Frank Dermody of Allegheny County voted for it. With or without his vote, the decision would have been the same.
He issued a statement Wednesday saying, "We respect the court's ruling on the plan, and I will work diligently with my colleagues on the commission to address the court's specific concerns as soon as those are made known."
Contact staff writer Tom Infield
at 610-313-8205 or email@example.com.