Proposition 8, a ballot measure approved by California voters that seeks to deny marriage to same-sex couples, is invalid, a coalition of legal and human rights groups has said.

The National Centre for Lesbian Rights (NCLR), the American Civil Liberties Union (ACLU), and Lambda Legal have filed a reply brief in the California Supreme Court, the next step in the lawsuit seeking to overturn Prop 8.

The measure was passed on November 4th with 52% of the vote, bringing to an end nearly six months of legal same-sex marriages in the state.

The new brief backs state Attorney General Jerry Brown’s position that certain fundamental rights, including the right to marry, are inalienable, cannot be put up for a popular vote and cannot be applied to invalidate existing marriages.

The state Supreme Court ruled in May that a previous ballot measure banning same-sex marriage was invalid.

The court said that treating people differently based on their sexual orientation violates the equal protection clause of the California Constitution.

More than 18,000 gay and lesbian couples married in California before November 4th.

Supporters of Prop 8 have asked the court to invalidate those marriages and uphold the democratic results of the November ballot.

“If Prop 8 is permitted to stand, it would be the first time an initiative has successfully been used to change the California Constitution to take way an existing right only from a historically targeted minority group,” said NCLR Legal Director Shannon Minter.

“Such a change would defeat the very purpose of a constitution and fundamentally alter the role of the courts in protecting minority rights.”

They argue that Proposition 8 is invalid because it seeks to eliminate a fundamental right only for a targeted minority, which cannot be done through the initiative process.

On November 19th the California Supreme Court granted review in the legal challenges to Proposition 8, and established an expedited briefing schedule, under which briefing will be completed in January 2009, with amicus curiae or “friend-of-the-court” briefs due on January 15.

Oral argument potentially could be held as early as March 2009.

Jennifer Pizer, Lambda Legal National Marriage Project Director, said that even if Prop 8 was valid it would not invalidate those same-sex marriages conducted before the November vote.