The ports of Los Angeles and Long Beach on Thursday filed legal papers outlining their opposition to legal efforts to block the ports’ controversial clean air program.

In addition, two large out-of-state freight haulers that are members of the industry group fighting the program have instead signaled that they’re now on board with the port’s initiative.

The American Trucking Association on July 28 filed suit to block implementation of the Clean Trucks Program. The ATA alleges the ports are attempting to impose restrictions on motor carriers that are tantamount to industry regulation, in violation of federal law.

A hearing on the preliminary injunction is scheduled for Sept. 8 in Los Angeles federal court.

The Los Angeles Times reported Friday that two Phoenix-based ATA members — Swift Transportation Co., with 37 major terminals in 26 states and Mexico, and the Knight Transportation Inc., with nearly half of its fleet consisting of 2008 model trucks have signed letters of intent to participate in the program, undercutting the association’s position.

ATA leaders had no immediate comment on the development.

In a statement Thursday, leaders of both ports challenged the relevancy of the statute the ATA cites in its suit. They also contend the injuries the association said its members would suffer do not rise to a level warranting a preliminary injunction.

“The ATA lawsuit directly attacks the ports’ efforts to dramatically reduce truck-related polution and improve the safety, security and efficiency of Port operations,” said Port of Long Beach Executive Director Richard Steinke. “We strongly believe that our plan is lawful and we will vigorously oppose any action that will delay the Clean Trucks Program.”

The ports plan to kick off their $2.2 billion Clean Trucks Program Oct. 1 by banning all pre-1989 rigs from the harbor. The effort, which would result in the replacement of roughly 16,800 short-haul trucks with cleaner burning models, is aimed at reducing diesel truck emissions by 80 percent.