But Roger Salazar, a political consultant who has been directing the effort to defeat the proposal, said that Mr. Holder’s statement should reinforce deep concerns about the initiative, including the way it was drafted and what he called inflated claims by its backers of what legalization might do.

“This is sort of a shot across the bow from the federal government: They’re saying that, ‘If this thing moves the way we think it is, we’re going to come after you guys,’ ” he said. “That gives California voters one more reason to take a deep breath.”

California’s becoming the first state to legalize marijuana for recreational use would provide a real-life test of theories that proponents of legalization have long pressed: That it would provide a new stream of revenues for government, cut down on drug-related violence and end a modern-day prohibition that effectively turns many citizens into lawbreakers.

As it is, no matter what voters or Mr. Holder do, marijuana use in California these days appears, for all practical purposes, all but legal.

Mr. Schwarzenegger signed legislation last month that made possession of an ounce of marijuana an infraction  it had previously been a misdemeanor  punishable by a $100 fine. Medical marijuana dispensaries are common in many parts of the state, and getting a prescription is hardly challenging. Baby boomers who had not smoked marijuana since college now speak openly at dinner parties of their “medical” experimentation with the drug. The smell of marijuana is hardly unusual at outdoor concerts at places like the Hollywood Bowl.

A Field Poll last month found that 50 percent of respondents said that marijuana should be legalized; that is up from 13 percent when the organization first asked the question in 1969. And 47 percent said they had smoked marijuana at least once, compared with 28 percent when the question was asked in 1975.

“This is the first generation of high school students where a majority of their parents have smoked marijuana,” said Ethan Nadelmann, the executive director of the Drug Policy Alliance, which has been pushing for passage of the initiative.