A successful vote in California would mean one in every six Americans lives in a state with legal marijuana sales, including the entire West Coast.
The initiative is promoted by a well-funded and politically connected coalition spearheaded by former Facebook president Sean Parker.
"Today marks a fresh start for California, as we prepare to replace the costly, harmful and ineffective system of prohibition with a safe, legal and responsible adult-use marijuana system that gets it right and completely pays for itself," Jason Kinney, a campaign spokesman, said in a statement.
It asks voters to allow people 21 and older to buy an ounce of marijuana and marijuana-infused products at licensed retail outlets and also grow up to six pot plants for personal recreational use.
Smoking weed would remain off-limits in places where tobacco use already is prohibited, including restaurants, bars and other enclosed public places.
Sales of both recreational pot and medical marijuana initially would be subject to a 15 percent excise tax. Cities and counties would retain the right to prohibit pot-related businesses and to impose their own fees and taxes.
State officials estimate the measure would raise as much as $1 billion per year in revenue and reduce public safety costs — for police, courts, jails and prisons — by tens of millions. Provisions of the initiative, which requires a simple majority vote to pass, would direct most proceeds to covering regulatory costs, research on the effects of legalization, environmental mitigation, substance abuse treatment and other purposes.
It has drawn support from the California NAACP, the California Medical Association and the California Democratic Party. Sponsors are promoting it as a civil rights issue, arguing that minority communities suffer a disproportionate share of drug crimes and arrests. They also say the initiative would make it harder for people under 21 to obtain pot and easier for police to crack down on illicit sales than it has been in the two decades since California became the first U.S. state to legalize medical marijuana.
Opponents include the California Republican Party and groups representing police chiefs and hospitals.
"The dangers of marijuana are pretty clear in terms of motorist safety, criminal activity, impacts on society," said Cory Salzillo, legislative director of the California State Sheriffs' Association. "We don't believe that decriminalization will upend the black market."
California voters rejected pot legalization by 7 percentage points in 2010, two years before western states began liberalizing their approach to pot. Colorado and Washington became the first states to allow recreational sales in 2012, followed two years later by Alaska and Oregon.
Initiatives allowing for casual use have qualified for November ballots in Nevada and Maine.