San Francisco ? Five months and thousands of weddings after California’s highest court sanctioned same-sex marriage, anxious eyes around the nation will closely follow voters Tuesday as they decide whether to turn back the clock.

Given the state’s size and influence, the vote on a constitutional amendment banning gay marriage has become a referendum on sexual orientation and civil rights. Both sides call it the “Gettysburg” of the power struggle between the gay rights movement and the Christian right, with the victors capturing momentum in other states.

“As California goes, so goes the nation,” Mayor Gavin Newsom boldly predicted at a City Hall celebration the day the state Supreme Court legalized marriages of gays and lesbians.

The race has tightened over the last six weeks and is expected to be close. A Field Poll released Friday found 49 percent of likely voters oppose the ban and 44 percent favor it. In mid-September, the measure was losing by 17 points.

“In the minds of many people, Proposition 8 is the most important thing nationally on the ballot,” said Tony Perkins, president of the Washington-based Family Research Council, which supports the measure. “We have survived bad presidents. But many, many are convinced we will not survive this redefinition of marriage.”

Religious and civil rights groups, wealthy philanthropists and middle-class donors have poured $69 million into campaigns for and against Proposition 8, making the initiative the most expensive election question this year outside the race for the White House. Almost $21 million has come from campaign contributors outside California.

Even the presidential candidates weighed in on Proposition 8: Sen. John McCain endorsed it and Sen. Barack Obama opposed it. Former President Bill Clinton recorded a telephone message that went to millions of California households Friday asking voters to defeat the measure.

The majority of opinion leaders in the state, including almost every major newspaper, the League of Women Voters, the state NAACP, and moderate politicians such as Gov. Arnold Schwarzenegger and U.S. Sen. Dianne Feinstein oppose the measure, which critics say unfairly denies one group a basic right.

Corporations that normally shy from contentious issues also have come out against it. The founders of Google, Yahoo and Adobe Systems took out a newspaper ad Friday encouraging Silicon Valley residents to reject it.

“This is the most intense and expensive social issues fight we have ever seen. And I think the real reason is because it’s very rare in American life (that) we have ever put existing rights on the ballot,” said Patrick Guerriero, a former leader of the gay Log Cabin Republicans who now directs the “No on 8” campaign.

But the measure’s opponents have found a formidable foe in the coalition of religious and social conservatives who sponsored the initiative. Since leaders of the Utah-based Church of Jesus Christ of Latter-day Saints appealed to members to back the ban, Mormon dollars and volunteers have streamed into California.

L. Whitney Clayton, the church’s liaison with the

Protect Marriage.com coalition, said the religious right and Mormons see a threat to the fundamental underpinnings of their faiths.

“The impact upon society over the long run is something that makes people very apprehensive,” Clayton said. “What will our children be taught in school? What will happen to the freedom of religion? What will people be able to preach and believe, and will they be able to do the things that they are accustomed to doing?”