BOISE - A federal judge has blocked a new state law that sought to strip labor unions of power, ruling the measure passed this year during a flurry of anti-union zeal in the Idaho Legislature likely conflicts with federal law.

The law passed with heavy Republican support during the 2011 session, despite an opinion from the Idaho Attorney General that it was illegal. Had it gone into effect July 1, it would have prohibited unions from using the dues they collect to subsidize members' wages as part of efforts to help union contractors submit winning bids on projects.

U.S. District Judge B. Lynn Winmill issued a preliminary injunction on Friday after unions sued, agreeing they had a good chance of succeeding. The unions argued the state law forbidding so-called "job-targeting programs" using union dues was unconstitutional because Idaho sought to pre-empt matters already governed by the federal National Labor Relations Board.

"This creates a risk of conflicting rulings from the state court and the Board, and threatens state interference with the NLRB's enforcement of national labor relations policy," Winmill wrote in his decision. "This Court has found that there exists a significant risk of overlapping jurisdiction between Idaho state courts and the National Labor Relations Board if the Fairness in Contracting Act becomes effective."

Winmill's decision was a natural consequence of an overreach by lawmakers intent on hamstringing Idaho unions further, regardless of whether it would withstand court muster, attorney James Piotrowski told The Associated Press.

"Congress designed the NLRB to resolve the legality of job targeting programs, not state legislatures," said Piotrowski, who represented the Idaho Building and Construction Trades Council AFL-CIO and Southwest Idaho Building and Construction Trades Council AFL-CIO. "There was a broad, anti-federal sentiment in the Legislature this year that was expressed a lot of ways, one of which was in this law, which even the attorney general told them was going to problematic because it conflicted with federal law."

A spokeswoman for Attorney General Lawrence Wasden's office said staff was reviewing Winmill's decision. Sen. John Goedde, R-Coeur d'Alene and the law's sponsor, couldn't be reached for comment Tuesday.

On Jan. 25, when the anti-union measure was making its way through the Idaho Legislature, the Idaho attorney general's office concluded it stood on shaky legal ground.

"The draft legislation carries with it a significant possibility for successful challenge," wrote Assistant Chief Deputy Brian Kane at the time. "The United States Supreme Court has made clear in a series of decisions that where conduct is 'actually' permitted by the LMRA, application of state laws regulating such conduct ... are pre-empted."

Just one Republican in the Idaho Senate opposed the measure, while three GOP legislators voted against it in the House.

Last month in Winmill's court, the attorney general, among other things, argued the injunction bid should be rejected because the U.S. Supreme Court in at least two other cases has found circumstances in which state law is not pre-empted, even if the conduct at issue is arguably protected or prohibited by the federal National Labor Relations Act.

Those exceptions, however, apply only if the alleged conduct is of "peripheral concern" to the federal law, something Winmill wrote Idaho's law clearly wasn't.

"It is evident that the Court could not characterize the conduct the Legislature seeks to regulate with the Fairness in Contract Act as a mere 'peripheral concern' to the NLRA because it involves activities that lie at the core of NLRA concerns: union activities seeking to protect employees' jobs and wages," the judge wrote.

The unions are also challenging the portion of the state law that prohibits so-called "project labor agreements" that require contractors to forge pacts with unionized workers as a condition of winning a government construction job. They have not asked Winmill to block that portion of the new law with a preliminary injunction.