(San Francisco) In one ad after another, voters in California and Maine were besieged with images of what would supposedly happen if same-sex marriage were legal: Students on a field trip to a lesbian wedding, elementary kids gobbling up books featuring gay couples, kindergartners learning about homosexuality from their teachers.

The punch-to-the gut claim has emerged as the latest tool in the ever-evolving playbook of same-sex marriage opponents, and the Achilles’ heel of the gay-marriage movement. Voters seem to be swayed by the notion that gay marriage will be a corrupting force among children, even though critics blasted the message as a blatantly misleading case of fear-mongering.

“It was very effective. It’s drawing on the fears of the unknown,” said Sandy Maisel, director of the Goldfarb Center for Public Affairs and Civic Engagement at Maine’s Colby College. “There’s no evidence that it’s going to happen, but there’s very clear evidence that it’s an effective campaign tactic.”

Gay marriage opponents discovered the effectiveness of the schools message in last year’s successful effort to pass Proposition 8 to outlaw gay marriage in California.

After signing up to lead the campaign, political consultants Frank Schubert and Jeff Flint knew they had a problem: Polls were showing that residents tended to not have much of a problem with gay relationships.

With the help of focus groups, surveys and ammunition unwittingly supplied by their opponents, Schubert and Flint soon found a new way to frame the issue, by focusing on education.

It was a departure from past elections when the issue was defined in simpler terms – that marriage is a sacred institution between a man and a woman. The various strategies have helped conservatives win 31 consecutive ballot initiatives on gay marriage.

“We bet the campaign on consequences, especially on education,” Schubert recalled in March when he and Flint were named the “public affairs team of the year” by the American Association of Political Consultants for their work in California. “Education from the beginning, while it was one of three consequences, it was the one that was the most emotionally charged and the most powerful.”

In California and Maine, gay marriage supporters countered the claims with spots featuring prominent elected officials – California’s chief of public instruction, Maine’s attorney general – who insisted that same-sex marriage had nothing to do with schools.

They also angrily denounced as deceptive the visuals the Sacramento team employed, including a Massachusetts couple who lost a lawsuit seeking parental consent before same-sex families are discussed in elementary classrooms.

But the response did not defuse the hot-button issue, advocates on both sides of the issue observe, in part because they failed to address what many parents knew to be true: Many public schools already have lessons that include references to gay families in the younger grades and confronting anti-gay discrimination for older students. Although the topics usually are broached in the context of appreciating diversity and tolerance, for some parents any discussion of gay people is too close to talking about gay sex.

“The trend that we are seeing is homosexuality is being promoted more and more in schools, and the increase in this is creating a hostile environment for kids with Christian or socially conservative viewpoints,” said Candi Cushman, education analyst for the Christian group Focus on the Family.

Cathy Renna, a public relations consultant in Washington who is married to a woman and has a 4-year-old daughter, said that equating references to gay parents with sex is “like saying that introducing someone’s mother and father to a class means you are talking about heterosexual sex.” But Renna agrees that same-sex marriage supporters need a different comeback to the kids-and-schools argument.

“This idea that gay people are coming to eat your children is a long-standing tactic of the right wing,” she said. “The response to those ads that not only has more truth, but more integrity, is that we live in a diverse world and our kids know that and it’s irresponsible for us not to talk about the world we live in in age-appropriate ways. Dismissing them as lies actually does a disservice not only to the people in our community, but to the public that knows better.”

In California, some gay rights groups want to try to repeal Proposition 8 at the ballot box next year. There has been talk about including language in the new measure that would state that nothing in it is meant to mandate the teaching of same-sex marriage in schools. Some gay rights advocates fear, though, that the wording could be used to undermine the way gay subjects are treated in schools now, said Chaz Lowe, founder of Yes! on Equality.

Melissa Murray, an assistant professor at the UC Berkeley Boalt Hall School of Law who researched the messages used in the Proposition 8 campaign, said gay marriage advocates underestimated how deeply Schubert and Flint’s carefully crafted schools message resonated with the public.

One reason it resonated so deeply is it changed the debate from one of equal rights to the equally cherished notion of individual rights, something gay activists should keep in mind as the marriage moves to other states, Murray said.

“Parents are always thinking about how do I keep unwanted influences out of my children’s lives, and it’s a lot harder to do that as a parent if that influence is the state,” Murray said. “That’s the fear they are tapping into. … and they are just going to keep repackaging it, because it works.”