A federal appeals court in San Francisco on Tuesday struck down California’s ban on same-sex marriage, ruling that the ballot initiative violated the equal-protection rights of gay and lesbian couples by withholding a marriage right they had already once enjoyed.

The decision sets the stage for a potential showdown at the US Supreme Court over an issue that has bitterly divided not just Californians but much of the nation.

The panel of the Ninth US Circuit Court of Appeals ruled 2 to 1 that California’s 2008 ballot initiative restricting marriage to between one man and one woman, violated the federal constitution by refusing same-sex couples the same marriage rights enjoyed by heterosexual couples.

“The People may not employ the initiative power to single out a disfavored group for unequal treatment and strip them … of a right as important as the right to marry,” wrote Judge Stephen Reinhardt in the majority decision.

“Proposition 8 serves no purpose, and has no effect, other than to lessen the status and human dignity of gays and lesbians in California, and to officially reclassify their relationships and families as inferior to those of opposite-sex couples,” Judge Reinhardt said. “The Constitution simply does not allow for laws of this sort.”

The decision represents an extraordinary use of judicial power to invalidate an act of grass-roots democracy. It dismisses the expressed judgment of seven million California voters who approved Prop. 8, and it undercuts a decision of the California Supreme Court that the ballot initiative was a valid amendment to the state’s constitution.

Reinhardt denounced the ballot initiative as “nothing more or less than a judgment about the worth and dignity of gays and lesbians as a class.”

The judge rejected claims by Prop. 8 proponents that the measure was a legitimate effort by the state to promote families with a mother and father in the same home raising their own children. He also pushed aside claims that the measure upheld tradition.

“Tradition alone is not justification for taking away a right that had already been granted,” he said.

Judge Michael Hawkins joined the majority opinion.

In a dissent, Judge N. Randy Smith said he was not convinced that the ballot initiative and constitutional amendment promoted no legitimate government interest. He suggested the measure could be upheld as an effort by the state to promote responsible procreation and optimal family partnerships of mothers and fathers raising their own children.

What happens next

Supporters of Prop. 8 are expected to appeal. They have two options. They can ask all active judges on the Ninth Circuit to re-hear the appeal, or they can file an appeal to the US Supreme Court.

The case has attracted substantial national interest because it was seen as an opportunity for the a federal appeals court to address a more fundamental question – whether gay and lesbian Americans enjoy a federal constitutional right to engage in same-sex marriage.

The US Supreme Court has not directly addressed that issue, but has suggested that there is no such right. Gay-rights advocates had hoped that the Ninth Circuit panel might become the first appeals court to challenge that position, forcing the high court to confront the issue.

Instead, the judges took a different approach, ruling that the effect of amending the state constitution to ban same-sex marriage after same-sex couples had already enjoyed that right amounted to unequal treatment, in violation of the Fourteenth Amendment’s equal protection clause.

Judge Reinhardt’s decision comes at a time when gay rights groups are fighting across the country in the courts, at the ballot box, and in public opinion forums for equal treatment with heterosexual couples in marriage. Groups favoring the traditional definition of marriage are fighting back.

Currently, six states and the District of Columbia authorize same-sex marriage. Thirty states, including California, have passed state constitutional amendments banning same-sex marriage. In addition, 37 states have passed statutes defining marriage as between one man and one woman.