The city of San Francisco asked a federal judge today for the right to become an official party in a lawsuit challenging California’s ban on same-sex marriage.

City Attorney Dennis Herrera said San Francisco is “singularly well-prepared to become a co-plaintiff” in the lawsuit filed in federal court in May by a lesbian couple from Berkeley and a gay couple from Burbank.

Herrera said the city has developed a “wealth of evidence” and a “unique public sector perspective” in previous state court battles over gay and lesbian marriage.

U.S. District Judge Vaughn Walker will hold a hearing in San Francisco on Aug. 19 on the city’s bid and on at least two requests by other groups to become parties, or intervenors, in the case.

One of the other requests was filed by three gay rights groups supporting same-sex marriage and the other was filed by the Campaign for California Families, which opposes it.

Walker has previously ruled that the sponsors of Proposition 8, a state ballot measure that banned same-sex marriage, can be an intervenor to defend the measure.

The federal lawsuit was filed in May shortly before the California Supreme Court upheld Proposition 8, which was approved by voters in the Nov. 4 election.

The initiative, a state constitutional amendment, overturned a ruling in which the state high court concluded last year that same-sex couples have a constitutional right to marry in California.

The federal lawsuit is sponsored by the recently formed American Foundation for Equal Rights, based in Los Angeles, and the two couples are represented by prominent attorneys Theodore Olson and David Boies.

Foundation spokesman Yusef Robb said he had no comment on the requests to intervene in the case. The foundation’s written response to the requests must be submitted to Walker by Aug. 7.

But Robb confirmed that foundation president Chad Griffin, a partner in a Los Angeles communications firm, wrote to three civil liberties groups on July 8 to say the foundation would “vigorously oppose” any attempts by them to intervene.

Griffin charged in the letter that the groups had been hostile to the strategy of filing a federal lawsuit and “sought to undermine this case even before it was filed.” He said allowing them to participate would lead to inefficiency and delays.

The three civil liberties groups are the American Civil Liberties Union, the National Center for Lesbian Rights and Lamdba Legal.

They are seeking to represent three gay community groups–Our Family Coalition, Lavender Seniors of the East Bay, and Parents, Families and Friends of Lesbians and Gays–as a party in the lawsuit.

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