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 nuptuals here.

"I've never stumped before, but I want to be a part of this," Bumgarner said. The retired insurance executive and devout Mormon 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 Mormon, 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.