A bare majority of California voters would continue to allow gay marriage, according to a new poll released Friday.

The Field Poll of 672 likely voters found that 51% oppose Proposition 8, which would amend the state Constitution to define marriage as only between a man and woman. Forty-two percent of voters support the November ballot measure.

Poll director Mark DiCamillo said the results indicate a substantial change among voters since 2000, when Proposition 22, a similar ballot measure, was approved with 61% of the vote.

Proposition 22 and other laws that discriminate on the basis of sexual orientation were found to be unconstitutional by the California Supreme Court in May, and gay couples began holding weddings last month.

DiCamillo predicted election results would not mirror those of Proposition 22.

“There has been a long-term [shift] in voter attitudes toward greater acceptance of same-sex marriage,” he said.

Proponents of Proposition 8 view the poll results differently.

“We think this bodes quite well for us,” said Jennifer Kerns, spokeswoman for the Protect Marriage campaign. She noted that a Field Poll released in May showed that 54% of Californians opposed Proposition 8, and said the new results “show the opposition has lost a few percentage points and indicates they are losing momentum.”

DiCamillo cautioned against comparing the results, noting that the more recent poll is of likely voters and is based on the actual ballot question, whereas the one in May was not as specifically targeted.

The campaigns have not yet begun to flood the airwaves in what is expected to be an expensive advertising effort. But both sides are diligently raising money.

Proposition 8 foes have raised at least $2.3 million, $900,000 of which has been donated in the last month. The Los Angeles Gay and Lesbian Service Center donated $200,000 recently, according to records submitted to the California secretary of state.

Backers of the measure have also raised at least $2.3 million, with $118,000 having been donated in the last month.

The Vineyard Group, a land developer in Arizona, gave $60,000 to support the measure. Members of the family that controls the firm were big donors to Republican Mitt Romney’s presidential campaign.

The full extent of the fundraising is not known. At this point in the campaign, state campaign finance law merely requires that proponents and opponents disclose donations of $5,000 or more.

More details will be disclosed at the end of the month when the committees release fundraising reports covering the first half of 2008.

 jessica.garrison@latimes.com

 dan.morain@latimes.com