SAN FRANCISCO -- California's Supreme Court declared yesterday that gay couples in the nation's biggest state can marry.

The decision injects an element of uncertainty into the presidential race, where so far the Iraq war and the sputtering economy have largely overshadowed social issues.

Same-sex couples could tie the knot in California in as little as a month. But the window could close soon after -- religious and social conservatives are pressing to put a constitutional amendment on the ballot in November that would undo the Supreme Court ruling and ban gay marriage.

In its 4-3 ruling, the Republican-dominated high court struck down state laws against same-sex marriage and said domestic partnerships that provide many of the rights and benefits of matrimony are not enough.

"In contrast to earlier times, our state now recognizes that an individual's capacity to establish a loving and long-term committed relationship with another person and responsibly to care for and raise children does not depend upon the individual's sexual orientation," Chief Justice Ronald George wrote for the majority.

Massachusetts is the only other state to legalize gay marriage. The California ruling is considered monumental by virtue of its 38 million population. The state has an estimated 92,000 same-sex couples.

Unlike Massachusetts, California has no residency requirement for obtaining a marriage license, meaning gays from around the country are likely to flock to the state to be wed, said Jennifer Pizer, a gay-rights attorney who worked on the case.

The ultimate reach of the ruling could be limited, however, because most states do not recognize gay marriages performed elsewhere. Nor does the federal government.

The conservative Alliance Defense Fund said it would ask the justices for a stay of the decision until after the fall election in hopes of adding California to the list of 26 states that have approved constitutional amendments banning same-sex marriage.

Opponents of gay marriage could also ask the high court to reconsider. If the court rejects such a request, same-sex couples could start getting married in 30 days, the time it typically takes for the justices' opinions to become final.

Republican Gov. Arnold Schwarzenegger, who has twice vetoed legislation that would have granted marriage to same-sex couples, said in a statement that he respected the court's decision and "will not support an amendment to the constitution that would overturn this state Supreme Court ruling."

Yesterday's ruling could alter the dynamics of the presidential race and state and congressional contests in California and beyond by causing a backlash among conservatives and drawing them to the polls in large numbers.

A spokesman for Republican John McCain, who opposes gay marriage, said the Arizona senator "doesn't believe judges should be making these decisions."

McCain sought to strike a delicate balance.

He "supports the right of the people of California to recognize marriage as a unique institution sanctioning the union between a man and a woman, just as he did in his home state of Arizona," his campaign said.

But McCain tried to maintain his long-held stance that the issue should be left to the states. He opposes a federal constitutional amendment banning same-sex unions.

The promised California ballot initiative -- and others in Arizona and Florida -- could help mobilize conservatives to turn out in the fall there and elsewhere, and, perhaps, boost McCain's prospects.

Gay marriage aside, five proposals related to other conservative issues -- abortion and race-based affirmative action -- have failed to even make state ballots.

Now, however, conservative activists will have a talking point as they work to rally their supporters.

Like McCain, Democratic rivals Barack Obama and Hillary Rodham Clinton say the marriage issue should be left to the states.