PORTLAND (AP) - Gay marriage has lost in every single state in which it has been put to a popular vote. Come Election Day, gay-rights supporters are hoping to make Maine the exception.

In a referendum that is being closely watched around the country and has drawn millions in out-of-state dollars, Maine voters will decide Tuesday whether to repeal a state law that would allow same-sex couples to marry.

If it is repealed, it will be another major defeat for the gay-rights movement, which saw voters in California put a stop to same-sex weddings there last year. A loss in Maine would be especially heartbreaking, given the way New England has been the region of the country most receptive to gay marriage.

The polls have been difficult to interpret. But both sides say the contest will be extremely close and will hinge on turnout, particularly among the 18-to-25-year-olds who went to the polls in great numbers last year to elect President Barack Obama.

"There's a knot in my stomach," said Steve Ryan of Buxton, who operates a property management business with his partner of 34 years, Jim Bishop. "We're very encouraged, and we're very worried at the same time."

Gay-marriage supporters have framed the issue as a matter of equality for all families, straight or gay. Opponents say that allowing same-sex couples to wed would be a dangerous social experiment and that Maine's domestic registry law could simply be bolstered to give gays additional legal rights.

"The stakes are very high for both sides. The gay marriage community has never won at the ballot box before on a straight up-or-down vote," said Frank Schubert, who coordinated the campaign to override California's highest court and repeal gay marriage there.

Over the past five years, 26 states have passed constitutional amendments limiting marriage to a man and a woman.

The Maine law was passed by the Legislature last spring but never went into effect because of a petition drive by opponents. Five other states have legalized gay marriage: Connecticut, Massachusetts, New Hampshire and Vermont - all in New England - and Iowa. All five of them did it by way of court or legislative action, not referendums.

"California was just a dress rehearsal for Maine," said Christian Potholm, a political science professor at Bowdoin College in Brunswick.

In related action Tuesday, voters in Washington state will decide whether to expand their domestic partnership law, and people in Kalamazoo, Mich., will consider whether to prohibit discrimination against gays.

In Portland, boisterous supporters of gay marriage - many of them students - far outnumbered the other side at the University of Southern Maine debate last week.

Enthusiasm for gay marriage has swept the campus, said Leigh Charest, 19. "People feel it's right and they want to do as much as they can," Charest said.

In addition to reaching out to young people, gay-marriage defenders have tried to make the campaign about the "Maine values" of personal freedom - the flinty, just-let-me-be attitude embodied by Maine's rugged lobstermen, loggers and outdoorsmen.

The pro-gay-marriage side has "very adeptly said this is not a campaign about telling people what they have to do. It's about allowing people the independence to do what they want to do. That's a basic, firm Maine value," said Sandy Maisel, director of the Goldfarb Center for Public Affairs and Civic Engagement at Colby College in Waterville.

Philip Spooner, an 87-year-old World War II medic and ambulance driver, became an Internet sensation thanks to his testimony in favor of gay marriage at a public hearing.

On the video, viewed more than a half-million times on YouTube, the retired truck driver and laundry operator and lifelong Republican from Biddeford said he raised four boys - three straight, one gay - and expected them to be treated the same.

"It takes all kinds of people to make the world go," he said. "It doesn't make sense that some people who love each other can marry and others can't, just because of who they are. This is what we fought for in World War II, that idea that we can be different and still be equal."

As in California, the National Organization for Marriage has been a major contributor, funneling $1.5 million to those fighting same-sex marriage in Maine.

The New Jersey-based organization has come under fire for refusing to release the names of its contributors as required by Maine law, saying the measure violates the First Amendment. The state ethics commission is investigating.