The court’s 4-to-3 decision striking down state laws that had limited marriages to unions between a man and a woman makes California only the second state, after Massachusetts, to allow same-sex marriages. The decision, which becomes effective in 30 days, is certain to play a role in the presidential campaign.

“In view of the substance and significance of the fundamental constitutional right to form a family relationship,” Chief Justice Ronald M. George wrote of marriage for the majority, “the California Constitution properly must be interpreted to guarantee this basic civil right to all Californians, whether gay or heterosexual, and to same-sex couples as well as to opposite-sex couples.”

California already has a strong domestic partnership law that gives gay and lesbian couples nearly all of the benefits and burdens of heterosexual marriage. The majority said that is not enough.

Given the historic, cultural, symbolic and constitutional significance of the concept of marriage, Chief Justice George wrote, the state cannot limit marriage to opposite-sex couples. The court left open the possibility that another terms could denote state-sanctioned unions so long as that term was used across the board.

The state’s ban on same-sex marriage was based on a law enacted by the Legislature in 1977 and a statewide initiative approved by the voters in 2000, both defining marriage as limited to unions between a man and a woman. The question before the court was whether those laws violate provisions of the state Constitution protecting equality and fundamental rights.

Conservative groups have proposed a new initiative, this one to amend the state constitution to ban same-sex marriage. If it is allowed onto the ballot in November and approved by the voters, Thursday’s decision would be overridden. The groups have gathered more than a million signatures on initiative petitions and submitted them to the state.

Justice Marvin R. Baxter, dissenting, said the majority had should have deferred to the state Legislature, which has in recent years increased legal protections for same-sex couples.

“But a bare majority of this court,” Justice Baxter wrote, “not satisfied with the pace of democratic change, now abruptly forestalls that process and substitutes, by judicial fiat, its own social policy views for those expressed by the people themselves.”

The California Legislature has twice passed bills allowing same-sex marriages, but they were vetoed by Gov. Arnold Schwarzenegger, who said they would overturn the 2000 referendum.

Mr. Schwarzenegger opposes the current ballot initiative seeking a constitutional ban on same-sex marriage. He said Thursday that he respected the court’s decision and would not support overturning it, according to The Associated Press.

In 2004, Mayor Gavin Newsom of San Francisco directed the county clerk to issue marriage licenses to gay and lesbian couples. Before the California Supreme Court halted the practice, more than 4,000 same-sex couples received marriage licenses in San Francisco.

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