SANTA CRUZ, California  -- Mexico's drug traffickers are likely to lose customers in America's largest pot consuming state if California legalizes marijuana, but they won't lose much money overall because California's residents already prefer to grow their own, according to a study released Tuesday.

That means the proposal on the state's November ballot to legalize marijuana also will do little to quell the drug gangs' violent and sophisticated organizations that generate billions of dollars a year, according to the study by the nonpartisan RAND Drug Policy Research Center.

Californians, who make up one-seventh of the U.S. marijuana market, already are farming marijuana at a much higher rate than in neighboring states and tend to buy domestic rather than smuggled marijuana, the study found.

Tuesday was harvest day on a medical marijuana farm in Northern California, where dozens of volunteers cut, trimmed and processed hundreds of shopping bags full of freshly snipped buds.

"We're already growing our own in California, and what happens in California so goeth the nation," said Valerie Corral, who helped found and runs a Santa Cruz medical marijuana collective. "Legalizing marijuana might undercut Mexican marijuana sales, but it isn't about to derail the cartels. It will just shift their focus."

California voters will decide next month whether to legalize and tax their own recreational use of marijuana. The measure is closely watched in Mexico, where more than 28,000 people have died in drug violence since Mexico's President Felipe Calderon launched his crackdown on organized crime in late 2006. Both Calderon and President Barack Obama agree the vast profits cartels collect in the U.S. -- estimated by federal authorities between $18 billion and $35 billion a year -- fuel drug wars south of the border.

RAND found that less than $2 billion of those profits come from marijuana and that losing the California marijuana market would cost cartels about $180 million -- or 3 percent -- of all the money they make exporting drugs to the U.S.

Mexican President Felipe Calderon said the California proposition is of critical importance.

"We are watching very closely, very closely, to what will happen in November. We are not certain what impact it will have on Mexico," he told The Associated Press in an interview last week.

Making even a slight dent in cartel profits makes legalization worth doing, said Drug Policy Alliance spokesman Stephen Gutwillig, who supports the Prop. 19 California ballot proposal.

"The bottom line is that creating any lawful, legitimate market for sales of marijuana to adults isn't good for the criminal syndicates that currently control this gigantic underground economy," Gutwillig said.

Proponents of the proposition say they want to lower prison costs and find new revenue from marijuana taxes, and that the measure could reduce violence associated with the illegal drug trade in California and Mexico.

The Obama administration disagrees, and U.S. drug czar Gil Kerlikowske told the AP that the new study backs them up.

"This report shows that despite the millions spent on marketing the idea, legalized marijuana won't reduce the revenue or violence generated by Mexican drug trafficking organizations," said Kerlikowske, head of the U.S. Office of National Drug Control Policy. "The bottom line is that increased access and availability to marijuana jeopardizes the health and safety of our citizens."

Some former law enforcement officials, however, said it's hard to imagine there wouldn't be major cartel profits at stake.

"It's ridiculous to claim that ending prohibition won't have a big financial impact on these violent criminals' bottom lines," said Stephen Downing, a supporter of Prop. 19 and a former Los Angeles Police Department deputy chief of police.

But the RAND study concludes the only way to cut into the cartels' profits would be the unlikely scenario of legal marijuana growers taking over cartel distribution elsewhere in the U.S. Under that scenario, Mexican drug trafficking organizations, currently providing at least half the marijuana in the U.S., would lose roughly 20 percent of their total drug export revenues. Their remaining profits from more lucrative drugs like cocaine and heroin would continue to flow.

"If that happens, then legalization could reduce some of the Mexican drug violence in the long run," said Beau Kilmer, the study's lead author and co-director of the RAND Drug Policy Research Center.

But the study authors said they don't believe the federal government will stand idly by if home-grown smugglers were to capture the entire national market now held by Mexico-sourced marijuana.

"It would be difficult not to notice that the quantities produced and perhaps even taxed were vastly larger than what is needed to supply the California market alone," said the study.

But some say it's already beginning to happen.

"Smuggling in the U.S will be easy, as marijuana can be shipped to consumers in other states through our mail system, said economist John Carnevale, a drug policy expert who has worked with the U.S. Office of National Drug Control Policy for three administrations. "There is anecdotal information that this is already occurring."

Former San Jose, California, police chief Joseph McNamara says the proposed law's key goal isn't aimed at resolving Mexico's drug violence, and questioned RAND's assumptions about marijuana use and sales.

"Can a state facing a $19 billion dollar deficit casually pass up a chance to tax a product that escapes taxation only because it is illegal?" he asked.