Sen. Dianne Feinstein, California’s senior U.S. senator, has lent her support to the campaign to defeat Proposition 19, the marijuana legalization measure on the state’s November ballot.

The prominent Democrat, first elected to the Senate in 1992, signed the ballot argument against the initiative. On Monday, she issued a statement through the opposition campaign calling the measure “a jumbled legal nightmare that will make our highways, our workplaces and our communities less safe.”

Roger Salazar, spokesman for Public Safety First, said the opposition committee sought Feinstein’s support.

“She’s one of the most respected figures in California,” he said. “She has a great history with law enforcement and dealing with this type of issue. We’re looking at a bipartisan effort.”

Proposition 19 would allow adults 21 and older to possess, grow and transport marijuana, and would allow cities and counties to regulate and tax commercial sales. Most of the state’s top elected officials and candidates for statewide office — from both major parties — are against the initiative.

Dale Sky Clare, a spokeswoman for Tax Cannabis 2010, the committee behind the measure, said it was not surprising that Feinstein and other statewide politicians opposed it.

“I’m just not putting a lot of faith in politicians to lead,” she said. “The voters have always led on this issue.”

Both campaigns have been collecting endorsements and touting them on their websites.

The recent unusual endorsement of the measure by the state NAACP led to the first real debate in the campaign. The NAACP saw it as a civil rights issue, noting that enforcement of marijuana laws disproportionately affects African Americans. The endorsement infuriated a group of black ministers who have organized to fight the initiative and who believe it will lead to more crime and addiction.

The pro-legalization ballot argument was signed by several former law enforcement officials, including retired Orange County Superior Court Judge James P. Gray and retired San Jose Police Chief Joseph D. McNamara. On the no side, Laura Dean-Mooney, president of Mothers Against Drunk Driving, joined Feinstein in signing the ballot argument.

In deciding to oppose the measure, Feinstein cited a recent report from Rand Corp., the Santa Monica-based research institute. The report concluded that about the only certain effects of the measure was that the price of marijuana would plummet and consumption would increase.

“In addition, there are too many unknown factors related to law enforcement and public safety,” Feinstein concluded.

john.hoeffel@latimes.com