SAN FRANCISCO — Gay marriage opponents pulled off another victory at the ballot box this week by using a tried-and-tested argument: Approve it and children will be taught homosexuality in school.

Voters seemed to be swayed by the message, both in Maine and in last year's gay marriage battle in California. And it has become an important part of the ever-evolving playbook of gay marriage opponents, who have now won 31 consecutive statewide ballot measures against the issue.

In Maine and California, voters were besieged with ad images of what would supposedly happen if same-sex marriage were legal: students going on a field trip to a lesbian wedding, elementary school kids reading books featuring gay couples, kindergartners learning about homosexuality from their teachers — all without any say from parents.

Critics assailed the messages as blatantly misleading fear-mongering.

"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 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 noticed that polls were showing voters 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, the two soon found a new way to frame the issue by focusing on education. It's not the first time gay marriage opponents have played the education card, but not until the California campaign did it became the preferred strategy. That is a departure from elections in recent years when the focus was almost solely on the argument that marriage is a sacred institution between a man and a woman

Schubert said he had an ah-ha moment in California when a focus group watched a campaign commercial featuring a Massachusetts couple who described how their 7-year-old son came home from school and explained that a man can marry another man, something he learned in a children's book.

One of the members of the focus group shook his head, and Schubert asked the moderator to inquire. The participant said he would be angry if something like that that happened to his kids.