The leader of the D.C. chapter of America's largest labor group said on Monday that Mayor Vincent Gray's "assault upon the collective bargaining process" was jeopardizing his relationship with the unions that have been vital to his political success.

"It's unbecoming of a city that prides itself on its relationship with its public unions," Josyln Williams, the president of the Metropolitan Washington Council of the AFL-CIO, told The Washington Examiner. "His endorsement from organized labor could very well be at risk."

The feud stems from a provision tucked into Gray's budget that asked Congress to allow the city to avoid paying about $36 million in overtime that firefighters earned years ago and is the subject of ongoing litigation.

"It's the ultimate hit," said Edward Smith, the president of the firefighters' union. "And to seek congressional action to do it and circumvent collective bargaining, I can't even put a name to it."

Williams said that Gray's policy mimicked that of officials like Wisconsin Gov. Scott Walker, who waged an intense battle with labor unions in the Badger State.

"This is a behavior that you would expect from the governor of Michigan or Ohio or Indiana or Wisconsin," he said.

The tactic has prompted other unions to hire experts "to see what might be hidden" in the budget, said Williams.

Gray told legislators that his plan was an effort "to protect the interests of District taxpayers," and his spokesman declined to comment about Williams' remarks except to say that the mayor has shown a "commitment" to labor.SClBFor Gray, who hasn't said whether he'll seek a second term, the dispute could have political ramifications.

Lawmakers will have to approve Gray's request, which could give the mayor's political rivals -- some of whom sit on the D.C. Council -- a chance to seem especially friendly to labor.

"If Williams doesn't get behind Gray, it may be that some of the member unions would be free to go out and pick and choose from the candidates," said political consultant Chuck Thies, who has informally advised Gray. Thies said Gray's re-election would be "extremely complicated" without labor's backing.

Just last week, Gray's relationship with labor groups appeared stout when Geo Johnson, a leading union negotiator, appeared at the mayor's side and said he'd endorse Gray.

But away from the television cameras, other labor leaders were pressing the case against the mayor, and Williams' coalition voted to condemn Gray's plan as "absolutely inappropriate."

Williams said that he hoped blame rested with "some second- or third-tier individual" because it was "hard to believe that this mayor would knowingly take an action that he knows would come back right into his face."