Should California voters approve Proposition 19, legalizing personal marijuana growth, use, and distribution on Nov. 2, the federal government says it will still prosecute violators of federal drug laws in the state.

On Friday, US Attorney General Eric Holder announced that the Obama administration would “vigorously enforce” federal drug laws against people who sell, distribute or grow marijuana for recreational use.

Holder sent a letter to nine former chiefs of the US Drug Enforcement Administration, saying in part:

"Let me state clearly that the Department of Justice strongly opposes Proposition 19. If passed, this legislation will greatly complicate federal drug enforcement efforts to the detriment of our citizens." Los Angeles County Sheriff Lee Baca hosted a news conference Friday morning to draw attention to the letter.

Prop. 19 would allow Californians 21 and older to grow up to 25 square feet of cannabis plants, and to possess up to an ounce of marijuana. It would also empower cities and counties to regulate marijuana cultivation and sales. Several municipalities are poised to do so if the law passes, with initiatives concerning taxation and regulation on the same ballot as Prop. 19.

Law professors and legal experts say the federal government will retain the right to prosecute federal drug offenses if Prop. 19 passes, but it's a matter of resources as to whether they will prosecute a wave of more confident growers and users.

“There is no question ... that California can pass a law saying it’s no longer illegal to have, grow, or smoke marijuana,” says Jesse Choper, a professor at the University of California, Berkeley’s Boalt School of Law. “But that has absolutely nothing to do with the continued existence of validity in enforcement of the federal statutes.”

He points out a 2004 challenge to the federal Controlled Substances Act (CSA) – Gonzales vs. Raich – in which the US Supreme Court ruled that under the commerce clause of the US Constitution, Congress may ban home-grown cannabis even where states approve its use for medicinal purposes.

The federal government can enforce its drug laws however it wants, says Erwin Chemerinsky, Dean of the Law School at University of California, Irvine. “It would not be that the US would have the initiative struck down, but it could go after those violating federal law.”

Still, Mr. Chemerinsky, Mr. Choper, and other experts agree that the federal government will likely not pursue prosecuting those found in possession of small amounts of marijuana, because doing so would be too costly.

“The question for the federal government will be how many resources they want to devote to the prosecution of people possessing marijuana,” says Jessica Levinson, an adjunct professor of law at Loyola Law School in Los Angeles.

More likely, says Ms. Levinson, is an injunction against Prop. 19, much like the one the filed against Arizona's controversial illegal immigrant law this summer. Los Angeles City Attorney Carmen Trutanich has already contacted the Department of Justice asking them to file an injunction against Prop. 19., she says.

As expected, opponents of Prop. 19 cheered Holder’s letter.

"This is yet another reason to oppose this poorly written initiative," says Roger Salazar, spokesman for the “No On 19” campaign. "The state will lose billions in federal funding, it will lead to more kids on drugs and more traffic accidents,” he says.

Marijuana advocates, on the other hand, say they have been down this road before and are not afraid of a crackdown. They liken their cause to alcohol prohibition, which took action from the states to push the federal government to change its policies.

"Passing Proposition 19 in California will undoubtedly kick-start a national conversation about changing our country's obviously failed marijuana prohibition policies,” says Tom Angell, spokesman for “Yes On 19.” So the passage of Prop. 19 will still be a victory, he says.

Some marijuana advocates are betting on the fact that the federal government will choose to use its limited resources elsewhere.

“The reality is that the federal government has neither the resources nor the political will to undertake ... enforcement responsibility for low-level marijuana offenses in California,” says Stephen Gutwillig, California director of the Drug Policy Alliance. "The federal government may criminalize marijuana, but it can’t force states to do so, and it can’t require states to enforce federal law.”

Will we see people arrested by federal authorities for buying "legal" marijuana at shops?

“Perhaps, but they don't have the resources to enforce this long-term,” says Dale Gieringer, president of California NORML. “More likely they will seek injunctions against cities and counties that try to legalize.”