Attorney General Eric Holder is warning that the federal government will not look the other way, as it has with medical marijuana, if voters next month make California the first state to legalize pot.
Marijuana is illegal under federal law, which drug agents will "vigorously enforce" against anyone carrying, growing or selling it, Holder said.
The comments in a letter to ex-federal drug enforcement chiefs were the attorney general's most direct statement yet against Proposition 19 and set 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.
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."
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.
The Justice Department remains committed to enforcing the Controlled Substances Act in all states, Holder said.
"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.
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.
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. They said legalizing pot presented the same threat to federal authority as Arizona's recent immigration law.
In that case, Justice Department lawyers filed a lawsuit to block the enforcement of the law, saying that it infringed on federal powers to regulate immigration and therefore violated the U.S. Constitution. The case is now before a federal appeals court.
Experts say the two situations are not the same.
If Arizona wants to crack down on illegal immigration more strictly than the federal government, the U.S. can act to prevent police in the state from enforcing the law, said Robert Mikos, a Vanderbilt University law professor who studies the conflicts between state and federal marijuana laws.
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," Mikos said.
"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, and it has expanded even more since Holder said last year that federal law enforcement would defer to state laws on using it for medicinal purposes.
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. "I really think the AG and the federal government could put our tax dollars to better use."