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A crowd gathers at a candle light vigil at City Hall for No on Proposition 8 in San Francisco on Wednesday, Nov. 5, 2008. More than two thousand supported stood together against Tuesday's passage of Prop 8, that eliminated the right of gay couples to marry. (David M. Barreda/Mercury News)

Some pollsters say it was previously undecided Catholics, jolted from their uncertainty by a Sunday sermon, who made the decisive tilt toward eliminating same-sex marriage in California.

Others think that African-Americans and Latinos, voting in overwhelming numbers on Tuesday both to elect Barack Obama and pass Proposition 8, made a crucial difference.

Still others say that despite months of polling that led many to believe Prop. 8 was expected to fail, a huge bloc of Californians had always shown themselves to be internally conflicted, and therefore unpredictable, on whether gays and lesbians should be able to marry.

"The bottom line is the public, the voters, are very closely divided on same-sex marriage today," said Mark Baldassare, president of the nonpartisan Public Policy Institute of California. "And when all was said and done, you just have to say the 'yes' side was just a little bit more persuasive than the 'no' side."

Polls by Baldassare's institute and other groups had indicated voter opinion weighted against Prop. 8, and so the measure's victory Tuesday left many experts scratching their heads.

Some had predicted that Obama's wide popularity in California, and his few public statements against Prop. 8, would work in favor of same-sex marriage supporters. Instead, the measure's proponents used Obama to their advantage, sending thousands of Bay Area residents a mailer that prominently featured the candidate and a quartet of local African-American pastors expressing their opposition to same-sex marriage.

In mid-October phone interviews, 52 percent of likely voters told the Public Policy Institute that they would vote no on Prop. 8 if the election were held that day. Only 44 percent said they would vote yes, and 4 percent said they did not know how they would vote.

But when asked a slightly different question — "Do you favor or oppose allowing gay and lesbian couples to be legally married?" — the same likely voters reversed their opinions. Forty-nine percent opposed allowing same-sex marriages, 47 percent favored allowing them and 4 percent did not know.

That contradiction, in which a slight change of semantics swayed voters from one conviction to another, seemed to be at the heart of California's most expensive and emotionally-charged election this year. Once they finally weighed in, nearly 5.4 million voters, or 52.5 percent, voted in favor of Prop. 8 and nearly 4.9 million, or 47.5 percent, voted against it.

Mark DiCamillo, director of the Field Poll, another group that had shown the measure losing, said the polls narrowed in recent weeks, but also may have been fooled by a last-minute switch of opinion by Catholic voters, who represent about 30 percent of the state's electorate.

"This is the same thing that happened in 2000," said DiCamillo, speaking of the last statewide initiative to ban same-sex marriage. "The Catholics seem to have moved to the 'yes' side in the final weekend."

DiCamillo bases his hunch on exit polls, number-crunching and personal experience. In 2000, he attended his family's Catholic Church the weekend before the election and heard his priest implore parishioners to vote for a proposition that prohibited California from recognizing same-sex marriages.

"The Catholics just don't move until the very end, until they're exhorted to do so," he said.

The Rev. Mark Wiesner, spokesman for the Oakland Catholic Diocese, said there was no directive to have priests throughout the East Bay church network express their support of Prop. 8 last weekend. Anything that happened would have been at the discretion of the local pastor, he said.

But DiCamillo said he believes that Sunday was "the big crescendo of the Yes campaign," and a changing Catholic electorate became a defining reason for the discrepancy between poll results and the actual vote. As of last week, 44 percent of Catholic voters interviewed by the Field Poll said they were supporting Prop. 8, and 48 percent said no. He said that according to exit polls conducted Tuesday, 64 percent of Catholics voted for Prop. 8 and 36 percent did not.

Exit polls from network television stations also revealed a wealth of other data that could have observers, and especially opponents of Prop. 8, looking back on the vote for a long time. A CNN poll said that 70 percent of African-American voters supported the measure, compared with 53 percent of Latinos and 49 percent of whites and Asians. But DiCamillo said there was not yet enough information to jump to too many conclusions, pointing out that African-Americans represent only about 6 percent of California voters.

"That's not a very big sample upon which to make broad conclusions or estimates," he said. With big margins of error, he said "you're just throwing darts at the board" until more conclusive studies happen in the coming days and weeks.

Reach Matt O'Brien at 925-977-8463 or mattobrien@bayareanewsgroup.com.