San Francisco ? The California Supreme Court ruling legalizing gay marriage will not be the last word.

California voters will almost certainly hold a referendum on a constitutional amendment banning same-sex marriage in November, and for the first time anywhere in the U.S., the vote will have a direct and immediate effect on gay couples waiting to tie the knot.

The amendment needs a simple majority to pass, and if the voters reject gay marriage, their decision will supersede the high court’s. There are signs the contest’s outcome will be close.

It will certainly be costly – the two sides say they plan to spend at least $25 million combined on the campaign.

“The people who want to defeat the amendment are going to have to work very hard to be successful – harder than the people who want the amendment to pass,” said Charles Gossett, a California State University-Pomona political scientist who has analyzed a decades worth of poll numbers on the issue. “But I don’t think its impossible.”

Though the state has a history of being on the vanguard of gay rights, California residents have polled slightly against same-sex rights in recent years.

The most recent polls, conducted in 2006 and 2007, found that 51 percent and 49 percent of survey respondents opposed making gay marriage legal, while 43 percent and 45 percent endorsed the idea.

Those numbers have remained virtually unchanged since former Gov. Gray Davis signed legislation in 2003 giving registered domestic partners the same rights and benefits as married spouses and since same-sex marriage became legal in Massachusetts in 2004, according to Gossett.

Proponents of the November initiative marriage think the court’s 4-3 ruling will hit closer to home and galvanize moderate voters who don’t mind gay couples entering into domestic partnerships, but want marriage reserved for a man and a woman.

“You may find even increased support from 2000,” when an anti-gay marriage referendum passed easily, said Andrew Pugno of the California Marriage Protection Act campaign. “With this court decision, the need for the marriage amendment is brought into clearer focus.”

Mindful of the defeat suffered by gay marriage opponents two years ago in Arizona – the only state where voters have rejected a marriage amendment – the sponsors deliberately decided against trying to simultaneously repeal domestic partner rights, which two-thirds of California voters support.

The secretary of state still must verify the initiative, a decision expected next month.

Groups from across the nation already are pledging resources to defeat or support the measure, similar to gay marriage bans enacted in 26 other states. Colorado-based Focus on the Family and the Democratic Congressional Committee both donated funds during the signature-gathering phase.

The California Conference of Catholic Bishops will not take a position on the measure until it qualifies for the ballot, but “it is obviously an issue they will support,” said spokeswoman Carol Hogan. The church’s position could be a key factor with Latino voters, who have been registering in greater numbers, Gossett said. The pope reiterated his opposition to gay unions on Friday.

Gossett also noted that survey respondents often give what they consider to be politically correct answers on social questions such as gay marriage, but record their true beliefs at the ballot box. A month before 61 percent of California voters approved the 2000 marriage ban, only 52 percent of likely voters told pollsters they favored the proposition