SAN FRANCISCO—Levi Strauss & Co. is putting its famous pockets behind defeating a ballot initiative that would outlaw same-sex marriage in California.

The San Francisco-based jeans maker said Thursday it will co-chair with Pacific Gas & Electric a group trying to drum up opposition to Proposition 8 in the business community.

Chief Executive Officer John Anderson said the move is consistent with Levi's long history of supporting civil rights causes.

The company ended racial segregation of its California factories in the 1940s and was the first Fortune 500 company to offer health benefits to the domestic partners of its unmarried employees.

In July, PG&E donated $250,000 to the campaign to defeat the gay marriage ban and announced it hoped to persuade other corporations to do the same. Because PG&E is a public utility, though, taking a controversial political stand is less risky for the company than it might be for businesses with greater competition.

Levi Strauss has pledged $25,000 to Equality for All, the coalition leading the No on 8 campaign, says company spokesman E.J. Bernacki. Robert Haas, the company's chairman emeritus, and his wife have given $100,000, Bernacki said.

Two labor unions, the California Council of Service Employees and the California Teachers Association, each have given $250,000. Telecommunications company AT&T has given $25,000.

The referendum seeks to amend the state constitution to overturn the May state Supreme Court decision that legalized same-sex marriage here. The amendment's sponsors so far have raised more money in the race than its opponents, who hope that recent $100,000 contributions from actor Brad Pitt and director Steven Spielberg will motivate Hollywood celebrities and moguls to give.

Also Thursday, a Southern California-based business advocacy group came out against Proposition 8. The board of the Valley Industry and Commerce Association, which represents businesses in the San Fernando Valley area of Los Angeles voted "by a significant margin" to oppose the measure, said President Stuart Waldman.

While a few board members argued against VICA taking a stand, maintaining that Proposition 8 was not a business issue, many more directors said it would cost them money and talented workers if the amendment passed, Waldman said.

"The most eloquent argument we got from an employer is they spend so much on human resources dealing with different benefits under domestic partnership rules versus married employees," he said.