Los Angeles area building owners tentatively agreed Wednesday not to block union attempts to organize local security guards, lifting the last big obstacle to the unionization of at least 5,000 guards in local office buildings, labor officials said.

The Building Owners and Managers Assn. of Greater Los Angeles agreed to remain neutral to overtures by the Service Employees International Union to the security guards, union officials said. The association also agreed to permit a "card check" organizing process under which owners recognize the union if a majority of employees sign pledge cards, the union said. Under those conditions, unions rarely lose.

UCLA professor Ruth Milkman, a labor historian, called the agreement "a very big breakthrough" that could embolden efforts to unionize security guards elsewhere in the country.

The organizing effort began in 2002 in Los Angeles, as part of an effort to organize security officers nationwide. The service employees union pledged to organize across the industry and win the acquiescence of all major security contractors and building owners before going forward.

The premise, union President Andy Stern said, is to create a level playing field where companies have similar labor costs and compete on the quality of their service instead of price.

The union that organized the security officers, SEIU Local 1877, represents janitors and is overwhelmingly made up of Latinos. Most security officers in Los Angeles are African American. Their new local would be called SEIU Security Officers United in Los Angeles.

"What's significant about this campaign is that it's a real bridge between the black and brown," Milkman said. "The security officers are working the very same buildings as the unionized janitors."

Representatives for the building owners' trade group could not be reached for comment.