SAN FRANCISCO (AP) — Both sides in the gay marriage debate will be watching California’s highest court Thursday to see if the nation’s biggest state goes the way of Massachusetts and legalizes same-sex marriage.

The California Supreme Court was scheduled to rule on a series of lawsuits seeking to overturn a voter-approved law that defines marriage as a union between a man and a woman.

If the court rules in favor of the plaintiffs, California could become the second state after Massachusetts where gay and lesbian residents can marry.

“What happens in California, either way, will have a huge impact around the nation. It will set the tone,” said Geoffrey Kors, executive director of the gay rights group Equality California.

Supporters and opponents of gay marriage predicted a number of possible outcomes from the California court’s seven justices, six of whom were appointed by Republican governors.

Like the top court in Massachusetts, they could hold that prohibiting same-sex couples from marrying constitutes unlawful discrimination and order state lawmakers to remedy the situation.

Less likely but still feasible, they could bypass the Legislature and simply strike the one man-one woman definition from the marriage statutes, according to Kors. In that instance, the soonest couples could start walking down the aisle would be in 30 days, the time it typically takes for Supreme Court opinions to become final, he said.

A majority of the justices could also join the top courts in four other states that have upheld gay marriage bans.

Such a decision would leave any subsequent changes in the hands of voters or the Legislature, which has twice passed laws to make gay marriage legal. Gov. Arnold Schwarzenegger vetoed them both times, citing the ban approved by voters in 2000.

“If California issues a decision legalizing same-sex marriage, it will reinvigorate the fight for same-sex marriage” nationally, said Jordan Lorence, an attorney with the conservative Alliance Defense Fund. “But if they affirm that marriage is for a man and a woman, then what has happened is that Massachusetts is leading a one-state parade.”

Lorence and Kors agreed that there is another, more nuanced option. The court could strike down the 2000 ban, known as Proposition 22, but give the Legislature the leeway of devising a solution that falls short of allowing marriage for all.

That’s what the New Jersey Supreme Court did in 2006. After that court ruled that gay couples should receive the same legal protections as husbands and wives, New Jersey legislators opted to allow same-sex couples to enter into civil unions, designed to give them spousal rights without marriage.

California already offers same-sex couples who register as domestic partners the same legal rights and responsibilities as married spouses, including the right to divorce and to sue for child support. It’s therefore unclear what additional relief state lawmakers could offer short of marriage if the court renders the existing ban unconstitutional.

A coalition of religious and social conservative groups is attempting to put a measure on the November ballot that would enshrine California’s current laws banning gay marriage in the state constitution.

The Secretary of State is expected to rule by the end of June whether the sponsors gathered enough signature to qualify the marriage amendment, similar to ones enacted in 26 other states.