They say, however, that that scenario is highly unlikely. “We do not believe that the federal government will stand idly by if California were to capture the entire national market now held by Mexico-sourced marijuana,” they wrote in the report, called “Reducing Drug Trafficking Revenues and Violence in Mexico: Would Legalizing Marijuana in California Help?”

The researchers said that if California’s legal pot were smuggled around the country, it could replace most Mexican marijuana sales, slicing more deeply into cartel revenues.

The report, released Tuesday by the Rand Corp., the non-partisan research institute in Santa Monica, estimates legalized marijuana could displace Mexican marijuana sold in California, but says that accounts for just 2% to 4% of the revenues gangs get from drug exports.

A new study concludes that Proposition 19, which would partially legalize marijuana in California , would do almost nothing to curtail violent Mexican drug organizations that ship the drug across the border, a finding that undermines one of the main arguments proponents have made.

Comparing the Mexican drug gangs to the mafia after alcohol prohibition, the researchers also say that they are likely to find other businesses, just as the mafia did to replace bootlegging. In the short term, they conclude, violence might even increase as gangs fight over smaller revenues.

Proposition 19 would allow cities and counties to authorize the cultivation and sale of marijuana. It’s unclear, even if the initiative passes, how many would do that. It’s also unclear whether the Obama administration would allow it, since marijuana is illegal under federal law. The researchers do not address those issues. The initiative would also allow people 21 and older to possess as much as an ounce and grow up to 25 square feet of marijuana.

The initiative has triggered a serious debate south of the border, where a four-year campaign against drug gangs has left 30,000 people dead. Last week, Mexican President Felipe Calderon stressed his opposition, saying that the United States has done too little to suppress consumption. But Calderon's predecessor, Vicente Fox, supports the initiative and has called for legalization in Mexico.

Ethan Nadelmann, executive director of the Drug Policy Alliance, said the initiative would not solve Mexico’s drug problems in the short term, but said, “When marijuana is legal, over time, the criminal organizations are going to lose all of their competitive advantage.”

Roger Salazar, a spokesman for the campaign against Proposition 19, said it would have little impact on cartels if it passed. “There’s going to be a tremendous opportunity to traffic in marijuana and make money in avoiding the taxes, so there’s still going to be a shady element,” he said.

The Rand report takes a harsh view of U.S. government estimates of the role marijuana plays in the revenues earned by cartels, dismissing the commonly cited claim that it makes up 60%. The researchers estimated marijuana revenues at between 15% and 26%. The higher estimate was published by the drug czar in 2006, but the researchers could find no documentation to support it.

“This 60% figure is a truly mythical number, one that appeared out of nowhere and that has acquired great authority,” they wrote. “This figure should not be taken seriously.”

The report also notes that U.S. government estimates of marijuana production “have long been inconsistent and sometimes implausible.” To illustrate the absurdity of one number, the researchers calculated regular users would need to smoke a joint every two hours they are awake.

The researchers suggest the government should start collecting better data. “Existing estimates about drug production and consumption are cryptic, inconsistent, and often impossible to verify,” they conclude.

Gil Kerlikowske, the drug czar, embraced the report’s conclusion that Proposition 19 would not dent the cartels. “At a time when drug use in America is on the rise,” he said, “we must focus our efforts on actions that will protect young people from the harms and consequences of illegal drug use instead of supporting initiatives that will make our nation’s drug problem worse.”

As part of their study, which they acknowledge is replete with uncertainties that could alter the results, the researchers had to do such calculations as determining the average weight of a joint: .46 grams.

The researchers also conclude that Mexican marijuana, which is lower in quality and lower in the main psychoactive ingredient than California-grown pot, has a U.S. market share between 40% and 67%.

Putting the wholesale price of Mexican marijuana at about $400 per pound, they determined that Mexican drug organizations make about $1.5 billion from exporting marijuana to the United States.

The researchers conclude that California marijuana, grown legally, could compete with the price of Mexican marijuana, even if it were taxed. And they note that California consumers would likely prefer the state’s pot, which is at least two times as potent. In an earlier report, Rand concluded that the price of marijuana would plunge about 80%, if it were legalized.

-- John Hoeffel