HARRISBURG — A Pennsylvania law designed to make it easier for organizations like the National Rifle Association to challenge cities’ firearms ordinances in court received a fatal blow Monday.

The Pennsylvania Supreme Court unanimously barred the enforcement of the law, which the NRA’s lobbying arm had called “the strongest firearms pre-emption statute in the country.”

The justices agreed with a lower court that packaging the measure into a bill on criminal penalties for theft of secondary metals, like wire or cable, violated a constitutional requirement that bills must be confined to one subject.

“This is the end of the road for this law,” said Sen. Daylin Leach, D-Montgomery, who sued along with several other Democratic lawmakers and the cities of Philadelphia, Pittsburgh and Lancaster. “This was not a close case.”

In a statement, Philadelphia Mayor Jim Kenney said the law’s intent had been to “intimidate municipalities in Pennsylvania.”

“This is a great victory for proper legislative procedure and for the ability of local governments to adopt common sense gun regulations without fear of financially crippling litigation,” Kenney said.

The lower Commonwealth Court threw it out last year, but not before it had spawned NRA lawsuits against Philadelphia, Pittsburgh and Lancaster, and scared some municipalities into repealing their firearms ordinances.

Under the law, gun owners no longer had to show they were harmed by an ordinance to challenge it. Instead, it let “membership organizations” like the NRA sue on behalf of any Pennsylvania member.

“As the Commonwealth Court suggested, creating a civil cause of action for persons affected by local gun regulations is simply too far afield from the definition of new offenses relating to the theft of secondary metal to be considered part of one subject” under the state constitution, Chief Justice Thomas Saylor wrote in a 14-page opinion Monday.

The lower court’s ruling had been appealed by the Legislature’s top two Republican lawmakers, Mike Turzai and Joe Scarnati.