More than a third of the members of the California State Legislature have filed a “friend of the court” brief in the state Supreme Court backing legal challenges to Proposition 8.

It aims to deny lesbian and gay couples the right to marry.

Prop 8 was placed on the ballot in response to a California Supreme Court ruling in May allowing same-sex marriages. It was passed by Californian voters on election day last week.

With 100% of votes at polling stations counted, 5,424,916 (52.4%) voted in favour of a constitutional definition of marriage being between a man and woman. 4,832,086 (47.6%) voted against.

More than 2m postal and absentee votes have yet to be counted. However, the campaign to stop Prop 8 has conceded defeat.

The 44 politicians, among them Senate President Pro Tempore Don Perata, Senate President Pro Tempore-elect Darrell Steinberg and Assembly Speaker Karen Bass, said that the ballot measure “eviscerates the judicial branch’s ability to uphold the fundamental rights of all Californians under equal protection clause” of the state constitution.

They claimed that if Proposition 8 takes effect, “this court will no longer be the final arbiter of the rights of minorities.

“Furthermore, treating Proposition 8 as a mere amendment would divest the Legislature of its constitutional authority to subject such a fundamental abrogation of the equal protection clause to its deliberative processes.”

Last week, even before all the voters from polling stations were counted, the American Civil Liberties Union, Lambda Legal and the National Centre for Lesbian Rights filed a writ petition before the California Supreme Court urging the court to invalidate Proposition 8.

The California state legislators have filed their brief in support of their case.

The petition charges that Proposition 8 is invalid because the initiative process was improperly used in an attempt to undo the constitution’s core commitment to equality for everyone by eliminating a fundamental right from just one group – lesbian and gay Californians.

Proposition 8 also “improperly attempts to prevent the courts from exercising their essential constitutional role of protecting the equal protection rights of minorities.”

According to the California Constitution, such radical changes to the organising principles of state government cannot be made by simple majority vote through the initiative process, but instead must, at a minimum, go through the state legislature first.

“The California Constitution itself sets out two ways to alter the document that sets the most basic rules about how state government works,” the groups said in a statement.

“Through the initiative process, voters can make relatively small changes to the constitution.

“But any measure that would change the underlying principles of the constitution must first be approved by the legislature before being submitted to the voters.

“That didn’t happen with Proposition 8, and that’s why it’s invalid.”

The lawsuit was filed in the California Supreme Court on behalf of Equality California and 6 same-sex couples who did not marry before last Tuesday’s election but would like to be able to marry now.

The groups said they are confident that the state will continue to honour the marriages of the 18,000 lesbian and gay couples who have already married in California.