Alarm bells are sounding among California same-sex marriage supporters after a new poll showed an apparent shift in voter sentiment in favor of Proposition 8.

That measure would restrict marriage to heterosexual couples, and end the same-sex nuptuals that started in June following a California State Supreme court decision.

California voters now favor Prop. 8 by 47 percent to 42 percent, according to a new CBS 5 poll, with a 3.6 percent margin of error.

What happened? To put it simply, the Yes on 8 campaign put out a more effective ad, and, with more money to run it, has been blanketing the state. Amazingly enough, younger voters now support the measure after initially opposing it.

Despite a Sept. 18 Field Poll that showed the measure failing by a wide margin, the No on 8 campaign’s internal polling showed that the race has always been competitive, and a movement toward support of the measure in recent days. That’s why those who’ve been working on the campaign have warned all along of complacency, perhaps born out in a lag in fund-raising.

I’m told that changes are afoot at the No on 8 campaign, with plans for a more centralized decision making rather than rule by an executive committee. I’m not sure whether a new ad strategy will be pursued, but there is pressure to make some changes.

Update: Equality California, which is among the groups trying defeat the measure, said today that Yes on 8 folks have raised $25.4 million, about $10 million more than they have.