In the background was the Democrats' feeling that the dominant Republicans on the commission had gerrymandered many districts to favor their candidates.
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."
