A surprise appearance by San Francisco Mayor Gavin Newsom in an anti-gay marriage ad is being credited for increased support for the measure.

A new CBS poll by its San Francisco affiliate shows Proposition 8 – the ballot initiative that would outlaw gay marriage in the state once again – winning. Eleven days ago, the same poll indicated the opposite.

According to the poll, voters now favor the gay marriage ban by a five-point margin, 42-to-47 percent.

The sudden change of heart is being credited to a Yes-on-8 television advertisement featuring an unfortunate sound bite from Newsom, which began airing last Monday.

The Yes-on-8 (Proposition 8) commercial begins with Newsom at a May 15th rally at City Hall, where he is seen celebrating the state Supreme Court's decision legalizing gay marriage.

“This door's wide open now. It's going to happen – whether you like it or not,” Newsom declares to a cheering crowd.

The reason for Newsom's inclusion in the video is obvious: He appears self-satisfied and smug as he declares, “Whether you like it or not.”

Producers of the 30-second spot immediately pounced on the cocky image of gay marriage's most ardent ally, using it freely throughout. When a female announcer says, “We don't have to accept this [gay marriage],” Newsom reappears and boisterously declares, “Whether you like it or not.”

The ad warns that without Proposition 8, people would be sued over personal beliefs, churches could loose their tax exemption, and gay marriage would be taught in public schools.

Gavin Newsom is often credited with opening gay marriage in California. In 2004, he ordered San Francisco clerks to issue marriage licenses to gay couples against state law. Those marriages were eventually invalidated by the state Supreme Court. But a May decision by the court found a 2000 voter-approved gay marriage ban unconstitutional. Since then thousands of gay couples have wed in the Golden State.

“It [Gavin Newsom's sound bite] showed the arrogance of this measure on the part of those and the four justices who essentially want to cram it down everyone's throat,” Tom Loarie of the Yes-on-8 campaign told KPIX.

The statewide poll conducted between October 4 and 5 among 670 likely voters has a margin of error of plus or minus 3.6 percent – making the gay marriage race too close to call.

The issue continues to trend similarly as it has in the past, with only young voters changing their minds. Voters who identified themselves as regular churchgoers were most in favor of the gay marriage ban, while the non-religious opposed it.