WASHINGTON — A California Supreme Court decision clearing the way for gay marriages in the state injects an element of uncertainty into a presidential race in which the Iraq war and the sputtering economy have largely overshadowed social issues.

Sen. John McCain, the GOP nominee-in-waiting whose position on the issue rankles the Republican Party's conservative base, sought to strike a delicate balance to the Thursday ruling.

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 in response. "John McCain doesn't believe judges should be making these decisions."

McCain rejected the will of the state's high court even as he tried to maintain his long-held stance that the issue should be left to the states. He suggested that he backs an effort by California's religious conservatives to put a constitutional amendment defining marriage as solely between a man and a woman on the November ballot.

The Arizona senator opposes gay marriage but, in a break with the GOP's right flank, he also opposes a federal constitutional amendment banning same-sex unions on grounds that states traditionally have decided the issue. McCain did work to ban gay marriage in Arizona, campaigning for a ballot measure there in 2006. The measure failed.

This year, there are indications that the GOP's conservative base is not nearly as energized as the Democrats' liberal base. If true, a California ballot initiative — and others in Arizona and Florida — could help mobilize dispirited conservatives to turn out in the fall there and elsewhere, and, perhaps, boost McCain's prospects.

Complicating McCain's position, his top ally in California, Gov. Arnold Schwarzenegger, said in a statement that he respected the court's decision and would uphold the ruling. But he also said: "I will not support an amendment to the constitution that would overturn this state Supreme Court ruling."

Like McCain, Democratic rivals Sen. Barack Obama and Sen. Hillary Rodham Clinton said the marriage issue should be left to the states, and they, too, seemed to tread carefully.

"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," the Illinois senator's campaign said.

Clinton's campaign said she "believes that gay and lesbian couples in committed relationships should have the same rights and responsibilities as all Americans and believes that civil unions are the best way to achieve this goal. As president, Hillary Clinton will work to ensure same-sex couples have access to these rights and responsibilities at the federal level. She has said and continues to believe that the issue of marriage should be left to the states."

In a victory for gay-rights advocates, the California court narrowly overturned a voter-approved ban on gay marriage and said domestic partnerships are not a substitute for marriage.

Some Democrats and gay-rights advocates rejoiced. House Speaker Nancy Pelosi, D-Calif., called the ruling "a significant milestone."

In turn, some Republicans and gay-marriage opponents vowed to press forward with their ballot initiative effort. Tony Perkins, the head of the Family Research Council, said, "It's outrageous that the court has overturned not only the historic definition of marriage, but the clear will of the people of California."

California's secretary of state is expected to rule by the end of June whether the sponsors have gathered enough signatures to put the anti-gay-marriage question on the ballot. If approved, it would trump the court's decision.

In 2004 and 2006, voters in more than 20 states approved similar measures, and conservative groups were extraordinarily active in mobilizing their rank-and-file to go to the polls to support the initiatives. Some analysts have suggested that such turnout activity helped lift President Bush to re-election over Democrat John Kerry four years ago. Others dispute the notion.