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.