SAN FRANCISCO —

The U.S. government will "vigorously enforce" federal laws against marijuana even if voters next month make California the first state to legalize pot, Attorney General Eric Holder says.

Holder's warning, contained in a letter to ex-federal drug enforcement chiefs, was his most direct statement yet against Proposition 19, and it sets up another showdown with California over marijuana if the measure passes.

With Prop 19 leading in the polls, the letter also raised questions about the extent to which federal drug agents would go into communities across the state to catch small-time users and dealers, or whether they even had the resources to do it.

If the ballot measure passes, the state would regulate recreational pot use. Adults could possess up to one ounce of the drug and grow small gardens on private property. Local governments would decide whether to allow and tax sales of the drug.

But Holder stressed that the Justice Department remains committed to enforcing the Controlled Substances Act in all states.

"We will vigorously enforce the CSA against those individuals and organizations that possess, manufacture or distribute marijuana for recreational use, even if such activities are permitted under state law," he wrote.

The letter was dated Wednesday and was obtained by The Associated Press.

Medical marijuana users and experts were skeptical, saying there was little the federal government could do to slow the march to legalization.

"This will be the new industry," said Chris Nelson, 24, who smokes pot to ease recurring back pain and was lined up outside a San Francisco dispensary. "It's taxable new income. So many tourists will flock here like they go to Napa. This will become the new Amsterdam."

Holder also said legalizing recreational marijuana would be a "significant impediment" to the government's joint efforts with state and local law enforcement to target drug traffickers, who often distribute pot alongside cocaine and other drugs.

The attorney general said the ballot measure's passage would "significantly undermine" efforts to keep California cites and towns safe.

Officials in Los Angeles County, where authorities have aggressively moved to tamp down on an explosion of medical marijuana dispensaries, vowed that they would still assist the federal government in drug investigations.

County Sheriff Lee Baca and District Attorney Steve Cooley said at a news conference that the law would be unenforceable because it is trumped by federal laws that prohibit marijuana cultivation and possession.

"We will continue as we are today regardless of whether it passes or doesn't pass," Baca said. His deputies don't and won't go after users in their homes, but public use of the drug will be targeted, he said.

Both gubernatorial candidates - Democrat Jerry Brown and Republican Meg Whitman - oppose Prop 19 and declined comment Friday.

The ex-Drug Enforcement Administration chiefs sent a letter to Holder in August calling on the Obama administration to sue California if Prop 19 passes.

If California prevents police from enforcing the stricter federal ban on marijuana, the Supreme Court has ruled that the federal government cannot order local law enforcement to act, he said.

It "is a very tough-sounding statement that the attorney general has issued, but it's more bark than bite," said Robert Mikos, a Vanderbilt University law professor who studies the conflicts between state and federal marijuana laws.

"The same factors that limited the federal government's influence over medical marijuana would probably have an even bigger influence over its impact on recreational marijuana," Mikos said, citing not enough agents to focus on small-time violators.

Federal drug agents have long concentrated on big-time drug traffickers and left street-level dealers and users to local and state law enforcement. As police departments began enforcing California's medical marijuana law, the DEA only sporadically jumped in to bust medical users and sellers that local law enforcement was no longer targeting.

Allen Hopper, a drug law reform expert at the American Civil Liberties Union in Northern California, predicted that federal agents would selectively crack down on marijuana growers and merchants instead of going after every Californian who uses pot.

"They don't have the resources to flood the state with DEA agents to be drug cops," he said.

Nearly all arrests for marijuana crimes are made at the state level. Of more than 847,000 marijuana-related arrests nationwide in 2008, for example, just over 6,300 suspects were booked by federal law enforcement, or fewer than 1 percent.

Consequently, the fight over legalization may end up the same way medical marijuana did, experts said.

When Californians approved their first-in-the-nation medical marijuana law in 1996, Clinton administration officials vowed a harsh crackdown. But nearly 15 years later, California's billion-dollar medical marijuana industry is thriving.

During the Bush administration, retail pot dispensaries across the state faced regular raids from federal anti-drug agents. Their owners were sometimes sentenced to decades in prison for drug trafficking. Yet the medical marijuana industry still grew.

Besides California, 13 other states and the District of Columbia have legalized medical marijuana in recent years.

At the San Francisco Medical Cannabis Club, where you can buy marijuana-filled carrot cake and lollipops, manager James Kyne said the federal government would just be continuing "an endless cycle" with little positive effect.

Holder "is opening a bigger can of worms," Kyne said.

Dear xxxxxx,

We just learned that Proposition 19, the ballot initiative to legalize and tax marijuana in California, was defeated today.

While this loss is disappointing, Proposition 19 has done more for the marijuana legalization movement than anything to date. This campaign has brought the failures of prohibition under a global spotlight and reframed the debate in our favor.

Proposition 19 has created the broadest coalition of organizations and individuals willing to fight for an end to the racist and wasteful war on marijuana this movement has ever seen. It helped to draw more attention to the prohibition related violence in Mexico. And it brought us closer to legalization than we've ever been before - a goal I believe we will reach in 2012.

In many ways, we did win tonight.

SSDP chapters throughout California worked tirelessly over the last 2 months to register voters, canvass their campuses and educate their communities about this initiative. Just as impressive were the actions our chapters at colleges around the country took to call California voters and urge them to vote yes on Prop 19. SSDP members alone made more than 15,000 phone calls to voters in California!

SSDP's national staff could not be more proud of the hard work our students put into this campaign. We also know that there is much more work to be done not only in California, but across the country.

Please help our students continue this important work by donating today.

Sincerely,

Aaron Houston, Executive Director
Students for Sensible Drug Policy