SAN FRANCISCO >> For the second time in six years, California voters will consider legalizing recreational use of marijuana.

This time, supporters of the move have much more financial backing and professional campaign help than they did in 2010.

And polls show Proposition 64 with more than the 50 percent of voter support needed to pass.

Silicon Valley billionaires and wealthy backers from the already legal medicinal marijuana industry are among the top financial supporters, contributing a combined $21 million.

Opponents have raised about $2.5 million, with $1.4 million coming from Pennsylvania anti-drug crusader Julie Schauer.

California voters rejected a similar measure in 2010 after campaign leaders struggled to raise money and support for the lengthy ballot measure that was hastily written by the owner of a small medicinal marijuana store.

Four states and Washington, D.C., have legalized recreational marijuana in recent years, and 25 states currently allow medicinal pot, including California.

The drug remains illegal under federal law.

People on both sides say passage of Proposition 64 would likely ignite similar movements in other states and exert significant pressure on federal authorities to reconsider the federal prohibition.

“As California goes, so goes the nation,” said University of California, Berkeley political science professor Alan Ross.

Proposition 64 would allow people 21 and older to legally possess up to an ounce of weed and grow six marijuana plants at home. Varying tax rates would be levied on sales, with the money deposited into the state’s Marijuana Tax Fund.

The measure also would allow cities and counties to pass their own regulations and taxes.

Most of the money would be spent on substance abuse education and treatment. Some would go to repair damage done to the environment by illegal marijuana growers. Funds would also be allocated to train police to detect when people are driving under the influence of pot.

California’s nonpartisan Legislative Analyst’s Office estimated that state could collect up to $1 billion in taxes a year.

“It’s a huge deal and it’s long overdue,” said Steven DeAngelo, a Proposition 64 supporter who owns one of the nation’s largest medicinal marijuana dispensaries.

Opponents argue the measure will open marijuana markets now dominated by small farmers to corporate interests and lead to children using the drug through pot-laced sweets, cookies and brownies.

“Proposition 64 favors the interests of wealthy corporations over the good of the everyday consumer, adopting policies that work against public health,” said Kevin Sabet, co-founder of the California-based nonprofit group Smart Approaches to Marijuana.

His group contributed $890,000 to the campaign to defeat Proposition 64.

Backers counter that Proposition 64 includes measures to protect small farmers and will further regulate a growing industry that is still largely illegal.

However, some medical marijuana advocates fear the supply of pot for patients could be threatened if Proposition 64 passes because of new regulations it would impose on growers.

Several law enforcement agencies have donated a combined $250,000 to defeat the measure, and U.S. Sen. Dianne Feinstein announced her opposition in July.

The 62-page ballot measure was crafted by political professionals and has the backing of many elected officials, including Lt. Gov. Gavin Newsom, a Democrat who is running for governor in 2018. Gov. Jerry Brown said he’s close to announcing his position.

Napster founder and early Facebook investor Sean Parker has contributed nearly $9 million to the legalization effort, which has also attracted sizable contributions from an organization backed by billionaire George Soros and another supported by Weedmaps, which rates pot stores throughout the state.