To blunt that possibility, gay leaders and Proposition 8 opponents have been sponsoring casual events at restaurants in traditionally black neighborhoods in Los Angeles, meeting with black clergy members and recruiting gay black couples to serve as spokespeople on panels and at house parties and church events.

“This is black people talking to black people,” said Ron Buckmire, the board president of the Barbara Jordan/Bayard Rustin Coalition, a gay rights group in Los Angeles. “We’re saying, ‘Gay people are black and black people are gay. And if you are voting conservative on an antigay ballot measure, you are hurting the black community.’ ”

Black voters account for 6 percent of likely voters in most statewide elections, according to the Public Policy Institute of California, while Hispanic voters make up about 15 percent. But taken together, those two groups could easily decide the election, people on both sides of the issue said.

“If the white Christian evangelic movement believes they can do it alone, I’ve got news for you,” said the Rev. Samuel Rodriguez, the president of the National Hispanic Christian Leadership Conference in Sacramento, which supports the measure. “They don’t have the sheer numbers to do it without the minority effort.”

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The Obama factor is just one potential element in the battle over Proposition 8.

Both sides said they expected to spend $20 million or more to help blanket airwaves. One advertisement by opponents shows a heterosexual bride on her way to the altar thwarted by various obstacles — a broken door, a clingy child — before the tagline: “What if you couldn’t marry the person you loved?”

Polls have shown Proposition 8 is trailing. A Field Poll of likely voters conducted last week found the measure was favored by 38 percent of voters and opposed by 55 percent. Mr. Obama, who has said he does not favor same-sex marriage, has stated his opposition to Proposition 8, calling the measure “divisive and discriminatory” in a letter to a gay Democratic club in San Francisco.

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But opponents are not declaring victory.

“We think there’s 15 to 20 percent that are still undecided on this issue,” said Geoff Kors, executive director of Equality California, which supports gay rights. “We do believe that if we can get our message out at least equal to the other side, we will win, but that’s a fund-raising issue.”

Mr. Kors said opponents of Proposition 8 had raised about $12 million so far.

Supporters of the proposition, which qualified for the ballot shortly after the Supreme Court decision, said they had raised about $15 million.

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Those donations include money from religious and conservative groups, including $1 million from the Knights of Columbus and $500,000 from the American Family Association, run by the Rev. Donald E. Wildmon. That group’s Web site includes a fund-raising video for Proposition 8 featuring a clip of the Rev. Martin Luther King Jr. while a speaker comments on the duty of black pastors to speak out in favor of Proposition 8.

Some supporters of the measure also say they sense a newfound enthusiasm in their ranks since Gov. Sarah Palin became the running mate of the Republican presidential nominee, Senator John McCain.

“I think Governor Palin has obviously energized social conservatives and religious conservatives and all types of conservatives,” said Bruce Hausknecht, judicial analyst with Focus on the Family Action, the lobbying arm of Focus on the Family, a conservative group that has spent nearly $450,000 on supporting Proposition 8. “And if that motivates more of them to get out to the ballot box than would have for John McCain by himself that has to benefit socially conservative issues like Prop. 8.”

The black community has long had a conflicted relationship with gay men and lesbians, Mr. Buckmire said, equal parts homophobia and denial.

“For too long, black people seemed to think there were no gay people around, especially black ministers,” Mr. Buckmire said. “They’d say the most insanely anti-gay things, and then the choir would come up and the choir is 50 percent gay.”

Still, the tendency of black voters to oppose gay marriage extends beyond religion. Patrick J. Egan, an assistant professor of politics at New York University who has studied black voting patterns on same-sex marriage, said black voters consistently polled much lower than white voters on approval for same-sex marriage, about 16 percentage points, even when religion was not a factor.