For the first time in more than 30 years, Californians are set to vote on a ballot initiative that would legalize the possession and cultivation of marijuana.

The measure, known as Proposition 19, has two distinct parts. The first would make it legal for anyone over age 21 to possess, share or transport up to one ounce of marijuana for personal use, as well as cultivate up to 25 square feet of marijuana plants.

The second part of the initiative would authorize cities and counties to regulate and tax commercial marijuana production and sales.

But the measure is expected to fail in the November vote. Pre-election polling shows the proposition losing ground among voters. According to a Los Angeles Times / University of Souther California poll, likely voters opposed the measure 51 percent to 39 percent in support.

Leading the opposition, predictably, are California's police and law enforcement. Los Angeles County Sheriff Lee Baca argues that the social and economic costs of Proposition 19 outweigh the economic benefits to the state.

Proponents of the measure observe that illegal marijuana is easily available to most anyone with an inclination to procure it and so, in their view, it is sensible for the state government to reap any potential reward. Further, they say, the state expends hundreds of millions of dollars annually to process marijuana offenders through the California court and prison systems. And, finally, they argue, the legalization of marijuana would reduce the threat of crime and violence caused by Mexican cartels shipping their product across the border.

Proposition 19 has not gotten much support from California's elected officials. Governor Arnold Schwarzenegger and every major candidate for statewide office oppose the measure.