Here's a very crude cut at an answer. Lots of polls have been done over the years about attitudes toward gays, and they all show a similar trend: people have become more gay friendly at the rate of about 1% per year. Here are some examples from Karlyn Bowman's 2006 roundup of historical poll data:
- Same-sex relations "not wrong": increased from 11% to 31% between 1973 and 2004.
- Gays should have equal employment rights: increased from 56% to 89% between 1977 and 2006.
- Gay marriage should be valid: increased from 27% to 39% between 1996 and 2005.
- Gays should have adoption rights: increased from 29% to 45% between 1994 and 2004.
In 2000 Californians voted to ban same-sex marriage by a margin of 63%-37%. If attitudes toward gay marriage have followed their historical pattern, about 9% more Californians are in favor of it this year, which means they'd still vote to ban it, but by the smaller margin of 54%-46%.
In other words: this is likely to be very close. These numbers have fairly big error bars attached to them, and it's also possible, especially in California, that attitudes toward gay marriage since 2000 have softened faster than in the past. Still, right now it looks to me like the odds are slightly stacked against those of us who favor same-sex marriage. This is going to be a very tough campaign.