FRESNO, Calif. — Michael Bumgarner says he's never campaigned for a political cause before, but his strong opposition to same-sex marriage has prompted him to join thousands of volunteers going door-to-door in support of a ballot initiative that would ban gay nuptials here.

"I've never stumped before, but I want to be a part of this," Bumgarner said. The retired insurance executive and devout member of The Church of Jesus Christ of Latter-day Saints said his late mother would "turn over in her grave" if she knew that gays and lesbians could marry.

With less than 11 weeks until Election Day, supporters of Proposition 8 are ramping up their field organization and refining their message as they seek to persuade California voters to shut the door on same-sex marriage. It's the first time voters will be asked to weigh in on the issue in either California or Massachusetts — the states where gays have won the right to wed.

An estimated 15,000 backers of the measure, most of them members of the LDS, Catholic and evangelical Christian churches, knocked on doors and distributed campaign literature to registered voters throughout the state this weekend and last, according to Jennifer Kerns, spokeswoman for the Yes on 8 campaign.

The initiative is a constitutional amendment, similar to ones already enacted in 26 other states, that would overturn the California Supreme Court decision that legalized same-sex marriage. It needs a simple majority of votes to pass.

Ron Prentice, director of the coalition of religious and social conservative groups that qualified the amendment for the November ballot, said the group has ordered 1 million yard signs and 1 million bumper stickers.

"Unless the people are angry, nothing will happen," Prentice said. "We are going to change the Constitution and say on Nov. 4, 'Judges, you can't touch this."'

For now, the campaign's goal is to identify supporters and voters who are unaware or haven't made up their minds about the measure, said Al Almendariz, a retired air traffic controller and a Mormon.

Almendariz led a team of five people canvassing a suburban neighborhood southeast of Sacramento on Saturday, and their script was concise. The volunteers told people who answered their doors they were with the Proposition 8 campaign, an effort that would define marriage as being between a man and a woman. They didn't mention same-sex marriage unless a resident brought it up.