In November, Californians will vote on a ballot measure to legalize, regulate and tax cannabis. A new Public Policy Institute of California (PDF) poll shows a very small plurality supports the measure. Forty-nine percent of likely voters favor legalizing cannabis while 48 percent oppose it.

Not surprisingly, younger voters and San Francisco Bay-area residents are more likely to support the measure. There’s a strong gender divide, with 54 percent of men favoring legalization and only 42 percent of women agreeing. Latinos have high opposition to legalization, with 62 percent opposed and 37 percent in favor.

The poll finds that among all adults, support for legalization is actually lower, with only 48 percent in favor and 49 percent opposed (sampling error is between two and three percent). This seems improbable, given that older Americans are more likely to turn out in midterm elections, and senior citizens are the group most strongly in favor of keeping marijuana criminalized.

Compared with earlier polls, the PPIC survey shows dramatically lower support for legalizing cannabis among all adults. In April, a SurveyUSA poll of California adults found 56 percent favoring, 42 percent opposing. A CBS News poll, also from April, showed that 55 percent of adults from Western states favor legalization, while only 41 percent oppose it. A Field Poll from April 2009 found that 56 percent of registered voters (PDF) in California favored legalization and taxation of marijuana for recreational use.

PPIC has a very strong track record in California, but there seems to be something counter-intuitive about the results when contrasting all likely voters with all adults. That part has the feeling of being an outlier.

This is the worst poll for cannabis legalization in California so far, but it still shows that likely voters slightly favor it. With a divided electorate, the ballot initiative could go either way in November. Its fate will depend heavily on the quality of the public outreach, framing and get-out-the-vote efforts from both sides.