American Apparel has hired a prominent union buster to address a contentious organizing campaign at its Los Angeles headquarters, The Post has learned.

The retailer, struggling with declining sales and a shrinking cash position, has, sources close to the situation said, tapped Cruz & Associates — whose Web site boasts that it “responds immediately and effectively to union activity.”

Cruz is led by Lupe Cruz, a former organizer for apparel union Unite Here.

“They have been capturing employees during their shifts for one to two hours at a clip,” according to Nativo Lopez, a trustee of the executive board of the General Brotherhood of Workers at American Apparel, referring to mandatory meetings the company is having with its workers.

GBofW is trying to organize American Apparel workers.

It’s the second time this year the retailer has hired a firm to address an aggressive labor campaign that since March has included weekly protests outside its sprawling manufacturing facility in downtown LA.

Its employees are complaining about reduced hours and pay, layoffs and the ouster in December of American Apparel’s founder and former chief executive Dov Charney. The workers have also been targeting American Apparel stores, gathering in groups of 50 to 100, handing brochures to customers about the labor strife, Lopez said.

Tensions are so high between employees and management that the LAPD have been called to the headquarters.

The company hired armed security guards in August to patrol its building and erected a 10-foot fence around the headquarters.

At a weekly protest on Sept. 23, loud music was being piped into the parking lot for the first time, two sources said.

“I can only surmise that they were playing the music to drown out the protesters,” said Lopez.

American Apparel and Lupe Cruz did not return calls for comment.

Also on Wednesday, American Apparel CEO Paula Schneider, who has been vilified at these protests, tried to curry favor with the rank-and-file by moving up half of the workers’ bonuses to Oct. 2 from its usual early year payout, according to a memo obtained by The Post.