San Francisco Mayor Gavin Newsom has called a Justice Department brief defending the federal Defense of Marriage Act (DOMA) a “big mistake.”

“I think it's a big mistake,” Newsom, who was in Los Angeles for the city's annual Gay Pride parade, told the Los Angeles Times.

The gay blogosphere erupted Friday on news the Obama administration was defending the 1996 law that allows states to ignore legal gay marriage performed in other states and defines marriage as a heterosexual union for federal agencies.

Newsom is often credited with opening gay marriage in California, a right lost last November with the narrow passage of a constitutional amendment that bans gay marriage, Proposition 8, and remains a strong advocate for granting gay and lesbian couples the right to marry. Last month, the state Supreme Court upheld the constitutionality of Proposition 8. Gay activists have vowed to repeal the ban in 2010.

“I'm concerned about some of the arguments being made by the Justice Department,” Los Angeles mayor Antonio Villaraigosa said.

During the campaign Obama called the law “abhorrent” and promised he would repeal the law.

“If elected, I would call on Congress to enact legislation that would repeal DOMA and ensure that the over 1,100 federal legal rights and benefits currently provided on the basis of marital status are extended to same-sex couples in civil unions and other legally recognized unions,” he told gay weekly The Washington Blade.

The move angered gay groups and bloggers who called it a broken promise, and took exception to the brief's anti-gay tone. Prominent blogger John Arovosis called the arguments “despicable, and gratuitously homophobic.”

“It reads as if it were written by one of George Bush's top political appointees,” Arovosis said in a post at “Obama didn't just argue a technicality about the case, he argued that DOMA is reasonable. That DOMA is constitutional. That DOMA wasn't motivated by anti-gay animus.”

The White House said it was duty-bound to enforce the laws of the land unless clearly unconstitutional.

“The President has said he wants to see a legislative repeal of the Defense of Marriage Act because it prevents LGBT [lesbian, gay, bisexual and transgender] couples from being granted equal rights and benefits,” White House spokesman Shin Inouye said.

“However, until Congress passes legislation repealing the law, the administration will continue to defend the statute when it is challenged in the justice system.”

The administration applies similar logic on the subject of “don't ask, don't tell,” the 1993 law that forbids open gay service in the military. The White House insists the president is committed to repeal of the law and is actively engaged in discussing the issue with top Pentagon brass, but refuses to issue an executive order to end or suspend discharges. Meanwhile, Pentagon officials say they are merely upholding the law.