In an interview, Andy Pugno, the lawyer for Protect Marriage, called Brown's argument "an astonishing theory." He added that he was "disappointed to see the attorney general fail to defend the will of the voters as the law instructs him to."
The two briefs also disagreed about the fate of the estimated 18,000 same-sex marriages performed before the election.
Brown argued that Proposition 8 was not written to be retroactive and that the marriages should remain valid. Protect Marriage argued that no same-sex marriages should any longer be recognized.
The Supreme Court justices have indicated they will hear arguments in the case as early as March, with a ruling expected later in the spring. Kenneth W. Starr, the former Whitewater prosecutor and U.S. solicitor general, plans to argue on behalf of Protect Marriage, the group said Friday.
Some opponents of gay marriage have said that if the court strikes down Proposition 8, they expect to see an effort to recall justices who vote against the measure.
The issue could also have an impact on Brown's political future. The attorney general, a former governor, has been exploring a run to return to that job in 2010. Among his potential opponents for the Democratic nomination are San Francisco Mayor Gavin Newsom, who is a hero to many gay rights advocates for his decision to begin same-sex marriages in his city in 2004.
The current legal chapter started because of those weddings. In a landmark ruling in May, the California Supreme Court ruled 4 to 3 that the guarantee of equal protection in the state Constitution required that same-sex marriage be treated the same as heterosexual marriage.
In response, foes of gay marriage mounted a well-organized campaign to amend the Constitution in an effort to put the question out of reach of the judges.
Passage of the ballot measure ignited widespread protests by gay rights groups, including boycotts of supporters of the measure.
The day after the election, gay rights advocates and several city governments, including those of San Francisco and Los Angeles, filed legal challenges.
They argued that the proposition should be rejected because it was a wholesale revision of the Constitution instead of a more limited amendment. A constitutional amendment can be passed by majority vote after being put on the ballot by a signature drive, but a revision can be placed on the ballot only by a two-thirds vote of the Legislature or a constitutional convention.
