1 p.m.SAN FRANCISCO — More California voters say they would oppose a November ballot initiative to ban same-sex marriage in the state’s constitution than would support it, according to a survey released Friday.

The Field Poll found that 51 percent of likely voters say they would vote against Proposition 8, while 42 percent say they would vote for it.

The poll shows a turnaround from 2000, when 61 percent of voters cast ballots in favor Proposition 22, which strengthened the state’s 1978 one-man, one-woman marriage law with the words “Only marriage between a man and a woman is valid or recognized in California.”

That vote came a year after the state Legislature enacted the first of a series of laws awarding spousal rights to domestic partners.

This year’s ballot initiative comes on the heels of a landmark California Supreme Court decision in May that granted marriage rights to same-sex couples.

Hundreds of gay and lesbian couples have flooded county clerk’s offices throughout the state to receive marriage licenses since the ruling took effect in June.

The poll found that Democrats and Republican voters were starkly divided on the issue. Sixty-three percent of Democrats said they would vote against the measure, while 68 percent of Republicans said they would vote for it. Nonpartisans were against the initiative 66 percent to 27 percent.

The poll found the greatest opposition to the ballot measure among voters living in California’s coastal counties and large cities, as well as among voters younger than 30. In the San Francisco Bay area, 67 percent of voters oppose Proposition 8, while only 26 percent approve.

The Field Poll surveyed 672 likely California voters during the week of July 8-14. It had a sampling error of plus or minus 3.9 percentage points.

The results also matched up to a Field Poll in May that asked Californians if they supported the Supreme Court’s legalization of gay marriage. That poll, which had a sampling error of plus or minus 3.2 percentage points, showed that 51 percent said they supported it, and 42 percent opposed it.