Saying Proposition 8 violates constitutionally protected liberties, Attorney General Jerry Brown on Friday asked the California Supreme Court to strike down the same-sex marriage ban, even as supporters filed a brief that would erase the legal recognition of couples married before Election Day.

In a brief filed with the high court, the state top’s lawyer argues for the first time that Proposition 8 should be invalidated, saying it is “inconsistent with the guarantees of individual liberty safeguarded” by the California Constitution. Brown had not taken a position on the measure until now.

“There are certain rights that are not to be subject to popular votes, otherwise they are not fundamental rights,” Brown said in an interview. “If every fundamental liberty can be stripped away by a majority vote, then it’s not a fundamental liberty.”

Proposition 8 supporters on Friday further pumped up the wattage of the coming legal confrontation by naming Kenneth W. Starr, the former U.S. solicitor general and independent counsel in the Whitewater investigation against then-President Bill Clinton. Starr, dean of the law school at Pepperdine University, will argue the Yes on 8 case before the Supreme Court, in arguments that could begin as soon as March.

An estimated 18,000 same-sex couples married in California after the Supreme Court’s ruling in May that struck down existing legal bans. Brown has said that Proposition 8 is not retroactive to those couples, and he reiterated that statement Friday, saying the marriages of same-sex couples who wed between June 16 and Election Day should remain valid in California, regardless of whether Proposition 8 is upheld.

Proposition 8 supporters acknowledge those marriages were legal before Election Day, and say they are not trying to “nullify” them now. They argue that the plain language of Proposition 8 — “Only marriage between a man and a woman is valid or recognized in California.” — means those marriages can no longer be recognized in California, although they would still be valid in other states where same-sex marriage is legal or recognized.

“Proposition 8 is in effect, and only marriage between a man and a woman is recognized,” said Andrew Pugno, general counsel for the Proposition 8 Legal Defense Fund. “It doesn’t say only a man and woman can get married. It says that only marriage between a man and a woman is recognized. It means what it says.”

The group’s brief says Proposition 8 “encompasses both pre-existing and later-created same-sex (and polygamous) marriages, whether performed in California or elsewhere. With crystal clarity, it declares that they are not valid or recognized in California.”

Supporters of same-sex marriage charged that the real intent was to attack gays and lesbians.

“In doing what they are doing now, they are showing the world what their true agenda is: to harm lesbian, gay and bisexual people,” said Geoff Kors, executive director of Equality California. “No one could think it’s morally acceptable to destroy a legally existing marriage.”

Proposition 8 opponents filed suit the day after the marriage ban passed, arguing that the initiative process was legally flawed. Brown had urged the Supreme Court to take the case, but by saying Friday that Proposition 8 should be invalidated, he aligned himself with civil rights groups and left Proposition 8 supporters legally isolated. Normally, the attorney general would defend existing state law.

The Supreme Court’s ruling in May held that the state constitution provides a right to marry that cannot be denied to same-sex couples. The brief filed by the attorney general on Friday argues that in order invalidate such a fundamental right, the court must determine that there is a compelling justification to do so, such as the protection of public health or safety.

But, Brown argues, the court’s ruling in May found that no such compelling justification exists, so Proposition 8 must be invalidated.

Brown compared his decision to oppose Proposition 8 to Attorney General Thomas C. Lynch’s decision to oppose Proposition 14, a 1964 constitutional initiative that overturned a state law that prohibited housing discrimination based on race.

“It’s unusual, but it happens,” Brown said of an attorney general opposing existing law. “Existing laws include the whole of the constitution, and just as Attorney General Lynch filed a brief calling for the invalidation of Proposition 14, we’re filing a brief here calling for the invalidation of Proposition 8.”

Mercury News Staff Writer Howard Mintz contributed to this story.

Contact Mike Swift at mswift@mercurynews.com or (408) 271-3648.