The state council of the Service Employees International Union, the largest labor union in California, has endorsed Proposition 19, the initiative on the November ballot that seeks to legalize marijuana.

The endorsement, announced Tuesday, could boost the campaign, which has not been able to raise enough money for television advertisements and is relying on grass-roots outreach.

The SEIU, which says it has more than 700,000 members in California, is a significant political force in state politics, although it is not clear how much money or muscle it will put toward passage of the measure.

In a letter to the campaign, union President Bill A. Lloyd wrote: “As you know, our primary objective in the 2010 election is targeting the top of the ticket. The lion’s share of our focus and resources are targeted at electing Jerry Brown as our next governor, but we look forward to joining you in any way we can to help pass Proposition 19.”

Lloyd said the union decided to back the initiative because it could help raise revenue to avoid cuts to healthcare, home care, education and services for children, families, the elderly and people with disabilities.

“These new revenues will help the state and local governments protect and invest in jobs we need to provide for our families,” he wrote.

The initiative, which allows people 21 and older to grow and possess marijuana, authorizes no taxes but allows cities and counties to approve the sale of marijuana and tax it.

Roger Salazar, a spokesman for the opposition campaign, noted that the initiative does not guarantee any new revenues and said, “It strikes me that they have been sold a bill of goods.”

But he added that he did not think the union would offer much help. “I think they’ve made it clear that they have higher priorities,” he said. “From the No on Prop. 19 side, my hope is that this is not much more than moral support.”

But Dan Rush, an official from United Food and Commercial Workers Local 5 who is working closely with the campaign, expects more than that.

The SEIU has 15 locals in California that represent workers in healthcare, long-term care, building services, and state and local government. Rush said he hopes the SEIU and other unions will contribute money and workers to operate phone banks and walk precincts.

Rush said the endorsement also could trigger an increase in contributions.

“I think that this is going to encourage major donors and show everyone that Proposition 19’s campaign is a viable campaign,” he said. “We are building an element of credibility and momentum for the campaign that is historic.”

The pro-legalization campaign has made labor a key component of its election strategy. Besides Rush’s local, the initiative also has won support from the council that oversees political activities for the UFCW in California, the Northern California council of the International Longshore and Warehouse Union, Communications Workers of America Local 9415 and Sign Displays and Allied Crafts Local 510.

“I’m confident that other major labor unions are going to be weighing in,” Rush said.

john.hoeffel@latimes.com