Calif. Senator Dianne Feinstein first raised her voice against Proposition 8 – the ballot initiative that seeks to forbid gay marriage in California – in September, when she said “I believe we should uphold the ability of our friends, neighbors, and co-workers who are gay and lesbian to enter into the contract of marriage,” in a press release. Tuesday, she committed herself fully to the cause by participating in a major pro-gay marriage ad by NO on Prop 8.

In the commercial, the senator speaks directly to voters.

“In my lifetime, I've seen discrimination. And I see it again in Proposition 8. Proposition 8 would be a terrible mistake for California. It changes our Constitution. Eliminates fundamental rights. And treats people differently under the law.”

“Proposition 8 is not about schools or our kids. It's about discrimination and we must always say NO to that.”

“No matter how you feel about marriage, vote against discrimination.”

“And vote NO on 8.”

A clearly elated No-On-8 campaign said they were planning a major state-wide buy for the spot.

“We are so moved to have the strong, trusted and independent voice of Senator Feinstein joining the chorus of those voting NO on 8,” said Patrick Guerriero, campaign director for No on Prop 8. “California voters know that Senator Feinstein speaks from the heart and it matters when she urges voters to reject this unfair initiative.”

The new ad is a political coup of sorts for gay marriage backers who have managed to achieve an impressive come-from-behind lead over gay marriage foes in the state.

A new poll released last Wednesday by the Public Policy Institute of California reports that 52% of likely voters surveyed opposed Proposition 8 while 44% supported it – a previous poll had showed the gay marriage ban winning, if only by the slight margin of 3 points.

And a new wave of last-minute donations have evened out – and possibly even pulled ahead – the pro-gay campaign's finances over supporters of the gay marriage ban. The No-On-8 campaign said it had raised $11 million in the first 21/2 weeks of October, while, the primary backer of the gay marriage ban, said it had raised only $2.4 million in October. Gay marriage in California has attracted a combined record-breaking $60 million to decide the issue.

Opponents of gay marriage say they will stop at nothing to enact the marriage prohibition in California, calling the issue a “must win” more important than the presidential election.

Feinstein is the senior U.S. Senator from California. It's been 30 years since she took over as San Francisco mayor after the murder of George Moscone and the city's first openly gay city supervisor, Harvey Milk. After serving two terms, she became the first woman from California to be elected to the United States Senate. The Democratic senator has reportedly shown interest in running for Governor of the Golden State in 2010.