PALO ALTO, Calif. — The California Supreme Court’s decision legalizing same-sex marriage could thrust the issue back into the national debate during this year’s presidential race, but in a way that could pose obstacles to both Senator Obama and Senator McCain.

The California court’s ruling yesterday by a 4–3 margin will increase pressure on both men to take a clear stance on a measure likely to go before California voters in November to amend the state constitution to ban gay marriage. The referendum would effectively overturn the court’s decision.

“Either Obama or McCain could come to California and grab the brass ring of the majority of voters, but I think only John McCain would consider doing that,” a leading opponent of same-sex marriage on the coast, Randy Thomasson of the Campaign for Children and Families, said.

Activists have gathered 1.1 million signatures to get the gay marriage ban on the ballot this fall. However, none of the major candidates for president seemed eager yesterday to stake out a position on the measure.

“Barack Obama has always believed that same-sex couples should enjoy equal rights under the law and he will continue to fight for civil unions as president. He respects the decision of the California Supreme Court and continues to believe that states should make their own decisions when it comes to the issue of marriage,” a spokesman for Mr. Obama, Thomas Vietor, said in a statement.

In his response to the court decision, Mr. McCain’s campaign denounced judicial activism and said he supported the idea of a referendum, but the statement stopped short of urging people to vote to limit marriage to its traditional definition.

“John McCain 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,” a spokesman for Mr. McCain, Tucker Bounds, said. “John McCain doesn’t believe judges should be making these decisions.”

The presumptive Republican nominee’s position on the same-sex marriage issue has failed to satisfy either side in the debate. In 2004, Mr. McCain delivered an impassioned speech against a federal constitutional amendment to ban gay marriage, decrying the proposal as an unnecessary abridgement of states’ rights. In 2006, the senator appeared in television ads urging voters in his home state to back a referendum to amend the Arizona constitution to ban gay marriage. Arizonans rejected the measure, 51% to 48%.

A political science professor who has studied the impact of cultural issues on voting, Alan Wolfe of Boston College, said he thinks the California measure could become a “huge problem” for the Republican nominee.

“If McCain endorses this, he’s in real danger with independents that Obama could attract away from him. If he doesn’t endorse it, he’s in real trouble with religious conservatives already suspicious of him,” Mr. Wolfe said.

The Democratic presidential candidates, Mr. Obama and Senator Clinton, also oppose a federal constitutional amendment and favor repealing part or all of a 1996 law which banned federal recognition of same-sex marriages, the Defense of Marriage Act.

Mr. Obama and Mrs. Clinton support robust civil-union protections for same-sex couples, but the hopefuls oppose calling such unions “marriages,” which is precisely the issue addressed yesterday by the California court.

“Obama and Clinton, if they are consistent with their stated position, they would have to defend the status quo in California and condemn this ruling. It will be interesting to see if they do that,” a spokesman for a social conservative group, Peter Sprague of the Family Research Council, said.

If Mr. McCain endorses the California gay marriage ban, he will find himself at odds with his most famous backer in the state, Governor Schwarzenegger, who has unequivocally denounced the proposed constitutional amendment. “I will always be there to fight against that,” the governor said. He also called the measure “a total waste of time.”

Mr. Schwarzenegger’s blunt remarks arguably put him to the left of Mr. Obama on the gay marriage issue.

One prominent member of the gay community, Rep. Barney Frank of Massachusetts, told The New York Sun that the political impact of the court ruling was being overstated. “I think this has no effect on the presidential race,” Mr. Frank said. In 2004, when 11 states passed ballot measures banning gay marriage, President Clinton often commented that “God, gays and guns” cost the Democrats the election.

“It turns out it wasn’t ‘God, gays and guns’ in 2004. It was Iraq and terrorism,” Mr. Frank said. “The 2006 election repudiated that…. No Democrats who voted against the same-sex marriage amendment lost.”

Same-sex marriage proponents in New York said yesterday that the California ruling would boost efforts to legalize the practice in the Empire State. “We are really energized to bring this home and make New York the third state in the country to recognize marriage equality,” Alan Van Capelle of the Empire State Pride Agenda said. In 2006, New York’s highest court ruled 4-2 that same-sex couples had no constitutional right to marry. “The California court basically went through point by point of the New York decision and picked it apart,” Mr. Van Capelle said. He said his group is working to end Republican control of the New York Senate, where the majority leader, Joseph Bruno, has vowed not to allow a vote on a gay marriage bill.

In a statement yesterday, Mayor Bloomberg welcomed the California ruling and said he will keep pressing for such legislation. “The right to marry is so essential to the maintenance of strong families in a civil society that it may not be denied based on sexual orientation,” he said.

The California court’s majority opinion yesterday held that the state could not justify denying gays the right to marry in part be cause it had already accorded them all of the important privileges and duties that go with marriage. “Retaining the traditional definition of marriage and affording same-sex couples only a separate and differently named family relationship will, as a realistic matter, impose appreciable harm on same-sex couples and their children,’ the court’s chief justice, Ronald George, wrote. “Providing a separate and distinct designation for same-sex couples may well have the effect of perpetuating a more general premise—now emphatically rejected by this state—that gay individuals and same sex couples are in some respects ‘second class citizens’ who may, under the law be treated from, and less favorably than, heterosexual individuals or opposite-sex couples.”

In dissent, Judge Mavin Baxter warned that the court’s decision opened the door to polygamy and incest.

“Who can say that, in ten, fifteen, or twenty years, an activist court might not rely on the majority’s analysis to conclude, on the basis of a perceived evolution in community values, that the laws prohibiting polygamous and incestuous marriages were no longer constitutionally justified?” he asked, quickly adding, “In no way do I equate same-sex unions with incestuous and polygamous relationships as a matter of social policy or social acceptance.”

Judge Baxter also accused the majority of all but ignoring a 2000 ballot measure in which California voters passed, 61% to 39%, a statute recognizing marriage as the union of one man and one woman.