The campaign for Proposition 19, the initiative on the November ballot that seeks to legalize marijuana, plans to announce Tuesday that it has won the endorsement of the state council of the Service Employees International Union, the largest labor union in California.

The endorsement could bring a boost to 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.

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.

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

The pro-legalization campaign has made labor a key component of its election strategy. Dan Rush, a politically connected official from United Food and Commercial Workers Local 5 has been working closely with the campaign.

“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.”

Besides Rush’s local, the initiative has won support from the council that oversees political activities for the UFCW in California and four other states. The Northern California council of the International Longshore & Warehouse Union, Communications Workers of America Local 9415 and Sign Displays & Allied Crafts Local 510 also signed on.

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

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 the other endorsing unions will contribute money and workers to operate phone banks and walk precincts.

-- John Hoeffel