“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.”

Image Joseph Barham, left, and Ariel Owens after their marriage in San Francisco on June 17. Credit... Peter DaSilva for The New York Times

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.

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.

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.