Holder's letter was made public Friday. Los Angeles County Sheriff Lee Baca is hosting a news conference at his headquarters Friday morning to draw attention to the letter.

The initiative on the Nov. 2 ballot would allow Californians 21 and older to grow up to 25 square feet and possess up to an ounce of marijuana. It also allows cities and counties to authorize cultivation and sales. Several cities, including Oakland, appear poised to do so if the law passes.

U.S. Atty. Gen. Eric H. Holder Jr., in a letter sent Wednesday to nine former chiefs of the U.S. Drug Enforcement Administration, wrote, "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."

The nation's top federal law enforcement official said the Obama administration would "vigorously enforce" drug laws against people who grow, distribute or sell marijuana for recreational use even if California voters pass a measure to legalize it.

Possession and sales of marijuana are illegal under the federal Controlled Substances Act. In his letter, Holder wrote: "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."

Dale Sky Jones, a spokeswoman for the Prop. 19 campaign, said the federal government is continuing to enforce "a failed policy."

"We're not necessarily surprised that the establishment is coming down on the side of the status quo. ... If the federal government is going to disregard the voters, this is a states' right issue."

President Obama has said that he is opposed to legalizing marijuana, but his administration has ended prosecutions of medical marijuana collectives and patients that abide by state laws, in effect ignoring the Controlled Substances Act. California was the first state to legalize medical marijuana in 1996, and agents from the Drug Enforcement Administration targeted the state's dispensaries and collectives for years. They have continued to bust large-scale growers.

The administration has come under criticism from the initiative's opponents for not doing enough to try to defeat it. Last week, Mexico's president, Felipe Calderon, chided the Obama administration. The nation's drug czar, Gil Kerlikowske, has spoken out about the initiative and said there is no doubt about where the administration's stands. On Thursday, his office issued a news release highlighting a survey released a month ago that showed teens are starting to use marijuana at an earlier age, 17 years old in 2009 as opposed to 17.8 years old in 2008.

Californians are split on the issue. Polls have consistently shown that marijuana legalization is supported by about half of the state's electorate.

-- John Hoeffel

Photo: U.S. Atty. General Eric H. Holder Jr. speaks at a news conference at the Justice Department in Washington, D.C. Credit: Getty Images

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Proposition 19: California's marijuana legalization debate