Camille Reyes used to be apolitical and irked by almost anyone ringing her doorbell to make a pitch. But these days, she's knocking on strangers' doors with missionary zeal to talk about one of the touchiest topics on the election agenda.

One of hundreds of volunteers canvassing house-to-house across Oregon, she is trying to persuade voters to defeat Measure 36 (search), a proposed state constitutional amendment prohibiting same-sex marriage.

Eleven states have such amendments on their Nov. 2 ballots, but only in Oregon and Michigan do gay-rights groups and their allies feel they have any realistic chance of defeating them. Were all 11 amendments to pass — a plausible outcome — it would be a sobering setback for activists nationwide who a year ago were celebrating a court order legalizing same-sex marriage in Massachusetts.

"So many hopes are pinned on Oregon," said Roey Thorpe, executive director of Basic Rights Oregon (search). "Winning in even one place is so different from losing everywhere."

The latest independent statewide poll in Oregon, released Thursday, showed support for the ban at 51 percent, with 40 percent opposed and the rest undecided.

In Michigan, most surveys also have shown the amendment winning — but it is the only state where one major poll, by Gallup, showed a majority against the ban. The "No" campaign's leader, Wendy Howell, said her side has gained support by warning that Michigan's proposal could force universities and public agencies to scrap existing domestic partner health benefits.

Initially, analysts predicted that all the marriage amendments would work in favor of President Bush by motivating religious conservatives to turn out at the polls. But in Oregon and Michigan, both crucial battleground states, political organizers now say the effect of the marriage debate on the presidential race could be negligible — or even tilt away from Bush.

"Oregonians are independent thinkers," said Tim Nashif, head of the Defense of Marriage Coalition (search) that collected signatures for the amendment. "An Oregonian would have absolutely no problem voting for John Kerry and for Measure 36."

Nashif's coalition has raised about $660,000, most of it from individuals and church groups inside Oregon. The amendment's opponents have raised nearly $1.1 million — including $500,000 from the National Gay and Lesbian Task Force (search).

"We'll have a bigger impact on voter turnout than our opposition," said task force organizer David Fleischer. "The net result in Oregon could turn out to boomerang on the right-wing, pro-Bush people who put this measure on the ballot."

Fleischer noted that Oregonians have defeated numerous local and statewide anti-gay-rights ballot measures over the past 16 years, and said his group is well-organized.

"Oregon voters begin with a fuller understanding of gay people's lives than in other states," he said. "The outcome probably will be extremely close."

Nashif, whose coalition has been distributing "One Man, One Woman" yard signs, dismissed the relevance of Oregon's previous gay-rights votes. "This is a mainstream issue — it's not a far-right issue," he said.

The manager of the "No" campaign, Aisling Coghlan, said more than 25,000 homes have been visited so far by a volunteer corps that includes many straight people as well as gays and lesbians like Camille Reyes.

Reyes had a door slammed in her face, and received a lecture on the sinfulness of homosexuality, but also has had uplifting encounters — one 8-year-old boy insisted on donating a dollar to the campaign.

"I was really nervous about canvassing," said Reyes, 30. "But this was such an important issue, I knew I needed to step outside my comfort zone."

Some prominent Oregon Republicans have endorsed the amendment; some leading Democrats, including Gov. Ted Kulongoski — who is not up for re-election — have publicly opposed it. But many candidates of both parties are, by and large, sidestepping the marriage debate for fear of alienating a sizable chunk of voters.

"It shows how close the race is," said Shauna Shindler Ballo, a "No" campaign spokeswoman.

In both the Oregon and Michigan, the state Republican parties have endorsed a ban on same-sex marriage, but have not trumpeted the issue. "We're not out there saying, 'Vote for Bush because of this,'" said Michigan GOP spokesman Matt Davis.

In addition to Oregon and Michigan, proposals limiting marriage to a man and woman are on the ballots in Arkansas, Georgia, Kentucky, Mississippi, Montana, North Dakota, Ohio, Oklahoma and Utah. Missouri and Louisiana voters overwhelmingly approved similar amendments earlier this year; Alaska, Hawaii, Nebraska and Nevada already had such amendments.

Gay marriage is not legal in any of the states where the issue is on the ballot, but amendment supporters say they want to guard against any future court rulings resembling the one in Massachusetts.

Nashif said gay activists want Oregon to be a West Coast outpost for gay marriage to complement Massachusetts in the East.

"They know they're behind the eight ball, that they could lose in all 11 states," said Nashif. He predicted that, following an anti-gay marriage sweep next month, a planned 2006 referendum in Massachusetts would become a "slam dunk" for those seeking to quash the court opinion allowing gay marriage.

Of all the states considering marriage amendments, Oregon is the only one in which gay and lesbian couples married during a flurry of ceremonies performed earlier this year by sympathetic local officials.

Basic Rights Oregon leader Roey Thorpe and her partner were among more than 3,000 same-sex couples who wed in March and April at the county courthouse in Portland. The scenario of Measure 36 prevailing dismays her.

"It would be very personal," Thorpe said. "It would be our marriage that got defeated."