HARRISBURG — The Pennsylvania Supreme Court on Friday explained its reasons for rejecting the state’s legislative redistricting plan, and a justice who disagreed with the ruling said this year’s races will have to be held under the existing, decade-old maps.

The 87-page majority opinion said those who challenged the Legislative Reapportionment Commission’s new map of 50 Senate and 203 House districts showed there were numerous splits of municipalities that were not absolutely necessary.

It was also critical of districts that were not sufficiently compact, likening one Senate district to a wishbone, another to a crooked finger, and a third to an iron cross.

The ruling, written by Chief Justice Ronald Castille, a Republican, and joined fully by the three Democrats on the seven-member court, said the challengers also showed the commission “could have easily achieved a substantially greater fidelity to all of the mandates in (the state constitution) — compactness, contiguity, and integrity of political subdivisions — yet the LRC did not do so in the Final Plan.”

The majority opinion reviewed in depth the legal and historical issues involved in drawing up district maps, and the court’s role in reviewing their legality.

“The constitutional commands and restrictions on the process exist precisely as a brake on the most overt of potential excesses and abuse,” Castille wrote. “Moreover, the restrictions recognize that communities indeed have shared interests for which they can more effectively advocate when they can act as a united body.”

The majority said challengers succeeded this time, where they failed under prior redistricting appeals, in part because they attacked the commission’s plan as a whole, and showed there were viable alternatives with fewer municipal divisions.

“The 2001 plan was not challenged as a whole, and like every other plan since 1971, it was not challenged with compelling, objective, concrete proof that a large number of political subdivision splits were not ‘absolutely necessary,’ ” Castille wrote.

The ruling specifically did not lay out a number of splits, or level of population variation, that would be acceptable.

“We . . . realize that the absence of certainty is a frustration for the LRC, a concern ably articulated by counsel,” Castille wrote. “But, that is often the case when constitutional principles are at work, and particularly when competing constitutional principles apply.”

Castille acknowledged that the decision to throw out the plan has created uncertainty in the election season, which is now under way, with candidates circulating nominating petitions for the April 24 primary.

Castille said the commission delayed in issuing its plan, which it approved 4-1 in mid-December.