Caught off-guard by a proposed L.A. city ordinance that surfaced Oct. 31 to limit the spread of big-box superstores that sell both general merchandise and groceries, retailers are lobbying furiously to stop the proposal and threatening legal action if it is passed.

Backers of the ordinance, introduced by City Councilwoman Jackie Goldberg, say its purpose is to head off traffic congestion that such stores tend to generate, to help preserve smaller neighborhood grocery stores, and prevent high-wage union jobs from disappearing in the face of the superstores, which typically employ non-union workers.

It would change zoning law to forbid big-box stores in excess of 150,000 square feet that also have at least 11,000 square feet set aside for food and grocery sales.

Retailers could still build such “superstores,” but would have to obtain a variance to do it.

The Goldberg ordinance is set to go before the city Planning Commission on Nov. 13, then possibly back to the City Council in the next few weeks. However, Goldberg, who is term-limited out and has been elected to the state Assembly, leaves office on Dec. 1. If it is not approved by then, its primary advocate might become Councilman Mike Hernandez, who co-authored the ordinance.

No such general merchandise/grocery superstores have been built within the Los Angeles city limits, and big-box retailers like Wal-Mart Stores Inc. and Kmart Corp. say they have no plans to build such stores in the immediate future in L.A.

Nonetheless, Wal-Mart and Kmart officials strenuously object to the ordinance. They say it goes far beyond the traditional city role of land-use planning by setting limits based on the mix of goods sold by the stores.

“They are not just trying to regulate the size of the box, but also what we sell inside the box,” said Robert McAdam, director of state and local government relations for Wal-Mart.

McAdam said the ordinance also limits Wal-Mart’s ability to site these “superstores” in L.A. in the future.

Kmart officials say they do have plans for new stores in L.A., and last month they announced that one would go into the old Sears department store building at Olympic Boulevard and Soto Street in East Los Angeles.

But spokeswoman Mary Lorencz said the company hasn’t yet determined the size of those stores or whether any of them will sell food items.