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In a room on the top floor of an evangelical church in San Diego, dozens of Christians are engaged in 40 days of round-the-clock praying and fasting. They are asking God to stop gay marriage in California.
"We believe it's a defining moment in American history," said Lou Engel of the evangelical group, The Call. "As California goes, so goes the whole nation. And in many ways, California is a leadership state for not just America but the whole world."
At a Unitarian church across town, Jan Garbosky and Bonny Russell, both retired educators, tied the knot after 20 years of partnership just a few weeks ago.
"We thought this would never happen in our lifetime, at our age," Garbosky said.
Religious groups across the nation -- including evangelicals, Catholics and Mormons -- have mobilized in support of Proposition 8, a ballot initiative that would overturn California's Supreme Court decision to legalize gay marriage.
If passed, the proposition would change the state's constitution, redefining marriage as exclusively between a man and a woman.
Since the Supreme Court's May decision, gay couples eagerly flew to California. Massachusetts and Connecticut also have legalized same-sex marriage.
The fight over California's Proposition 8 has become one of the most expensive campaigns in the country -- second only to the presidential race.
Groups on both sides of the issue have thrown millions into the race. Supporters include Focus on the Family, the Knights of Columbus and members of the Mormon church. Opponents include Hollywood celebrity donors like Brad Pitt, Stephen Spielberg and Ellen DeGeneres.
At least one prominent evangelical has been quoted as saying that the Proposition 8 campaign is more important than the presidential race.
Many Christians believe that scripture prohibits homosexuality. If gay marriage is allowed to stand, some evangelicals suggest, it would force churches to marry gays, force schools to teach gay marriage, settling off a snowball effect and opening the door to pedophilia and bestiality.
"A person could say, 'I love my dog, why should we not be married?'" said Anna Good, a member of the Skyline Westlean Church.
But the opposition says these arguments are spurious.
"We live in a democracy, not a theocracy," said Garbosky. "I'm just amazed that anybody of faith would say [to] take away somebody else's rights."
Polls show that the race for the proposition is tight.
Both sides say a loss would be devastating, but that the fight will not end with Tuesday's vote.