Trevor Hughes

USA TODAY

Napster co-founder and former Facebook president Sean Parker donated $1 million to an effort to legalize recreational marijuana in California.

Parker is the single largest contributor to the ballot measure, which likely will go before voters in November, according to the California Secretary of State’s office. Backers are collecting the 365,880 signatures needed to qualify for the ballot. They have received $1.25 million from other donors, including Weedmaps and the Drug Policy Action committee. The money helps pay for people to collect signatures and for advertising to generate public support.

The ballot measure dovetails with and builds on newly passed California legislation establishing basic ground rules for legal marijuana in the Golden State.

Calif. legal pot efforts get push with Sean Parker-backed measure

The Parker measure, as it’s known by marijuana industry, is supported by the Drug Policy Alliance, the Marijuana Policy Project and the California Cannabis Industry Association, all major players in the legalization movement.

"Cannabis is absolutely ingrained in the Silicon Valley culture,” said Emily Paxhia, a founding partner with Poseidon Asset Management, a hedge fund that invests in marijuana and hemp businesses. Poseidon is backing the legalization measure to which Parker has donated.

Four other states permit recreational cannabis use, but advocates say California’s sheer size and position as a cultural trendsetter for the rest of the country means any decision by its voters could accelerate legalization elsewhere.

The stakes are huge. Experts say legal marijuana could be worth billions for California, not just through taxes, but through a new network of licensed cultivation, distribution and testing facilities, all of which will need employees, construction workers and equipment.

If passed, the measure would create a new Bureau of Marijuana Control, require growers and sellers to pay taxes, and establish stiff penalties for anyone caught illegally diverting water, an aspect popular with environmentalists. It would tighten the state’s comparatively lax medical marijuana system and bar use by anyone younger than 21.

The measure would allow adults to possess up to an ounce of marijuana at a time and grow six plants without needing a license. It also would bar public consumption, require quality and contamination testing and grant business licensing priority to recognized medical marijuana producers. Giving preference to established medical marijuana industry players means they'll get a jumpstart on their competition.

Calif. pot battle gears back up; tech money seen taking a role

A similar effort fell short of reaching the ballot in 2014 because organizers say they lacked the funding to gather enough signatures. A different plan made it to the ballot in 2010 but was rejected by voters.

Parker, who gave $100,000 to that effort, chairs a self-named philanthropic organization that focuses on civic engagement, life sciences and global health. He's been open about using his wealth to fund social causes and frequently donates to California politicians and ballot committees.

Parker couldn't be reached immediately for comment.