Opposition to a California ballot measure to ban same-sex marriage is mounting following Attorney General Jerry Brown's move to change the language on the initiative, according to a Field Poll to be released today.

The poll found that just 38 percent of likely voters support the measure, while 55 percent intend to vote no. That compares with 42 percent in support and 51 percent opposed in July.

Brown amended the Proposition 8 summary language after the state Supreme Court's decision on May 15 to overturn California's previous ban on same-sex marriage.

The pollsters found the amended language played a role in that growing opposition, especially among the 30 percent of likely voters interviewed who had never heard of Prop. 8.

Those voters were much more likely to oppose the measure when read Brown's wording (58 percent against it and 30 percent for it) than those in the same category who were read the old version of Prop. 8 (42 percent against and 37 percent for it), according to the Field Poll.

The Brown language reads, in part: "Eliminates the right of same-sex couples to marry." The original version read, in part: "Limit on marriage."

Brown's revision makes it clear that voters are taking away someone's rights and that made the difference, said Field Poll Director Mark DiCamillo.

"People are generally in favor of rights for individuals," said DiCamillo. "So eliminating a right has a somewhat negative connotation. It's pulling people who weren't aware of the initiative more to the no side."

Prop. 8 supporters sued unsuccessfully this summer to block Brown's changes, saying he was trying to stack the deck against the Nov. 4 ballot measure.

The new Field Poll just confirms that, said Jennifer Kerns, spokeswoman for Yes on Proposition 8.

"The figures are not surprising given Attorney General Jerry Brown's latest attempts to influence the elections," she said, adding that the Field Poll historically underestimates support for banning same-sex marriage.

And she also believes that polled voters may tell interviewers they support same-sex marriage but will "vote their conscience" in the ballot box.

"There's no doubt the views on same-sex marriage have softened a bit over the last eight years," she said. "It's our position that a majority of Californians still support marriage as between one man and one woman."

Opponents of Prop. 8 said the Field Poll is heartening.

"The proponents are trying to eliminate a constitutional right from one group of people and treat them differently," said Geoff Kors, executive director for Equality California.

"Once voters know the truth, it appears they are moving in the direction of voting no."

As of a week ago, supporters of Prop. 8 reported contributions of $14.7 million and opponents had reported $13.5 million. Actor Brad Pitt donated $100,000 on Wednesday to fight Prop. 8.

"Because no one has the right to deny another their life even though they disagree with it, because everyone has the right to live the life they so desire if it doesn't harm another, and because discrimination has no place in America, my vote will be for equality and against Proposition 8," Pitt said in a statement.

The Field Poll found that Democrats oppose the initiative nearly 4-1 (75 to 20 percent). Conservatives support Prop. 8 by a greater than 3-1 ratio (72 to 21 percent). Likely voters who live in coastal counties or those touching San Francisco Bay oppose Prop. 8 57 to 36 percent.

Protestants support the measure 52 to 40 percent, according to the poll, while a majority of Catholics (55 percent) and those affiliated with other religions or who have no preference (71 percent) plan to vote no. Evangelical Christians back the initiative 60 to 34 percent.

The poll is based on a telephone survey of 830 likely voters, taken between Sept. 5 and 14. There is a margin of error of plus or minus 3.5 percentage points.