The 700,000-member Service Employees International Union California has 15 locals representing workers in health care, building services and state and local government.
The possibility of members operating phone banks, walking precincts and raising money makes the union a political powerhouse in the state and a strong ally for the Proposition 19 campaign.
However, the endorsement letter from SEIU California president Bill Lloyd left unclear how much time and money the union would provide.
Most union resources will go toward supporting Jerry Brown in the governor's race, Lloyd said, although he vowed members would help Proposition 19 any way they can.
Revenue raised by legalizing marijuana could help the state preserve jobs and avoid cuts to health care, home care, education and other services, he added.
Proposition 19 calls for allowing adults to possess up to one ounce of marijuana. Local governments could tax its sales.
The campaign has won the support of several other labor groups, including the Northern California council of the International Longshore & Warehouse Union and Communications Workers of America Local 9415.
The measure also has the endorsement of the NAACP California chapter. President Alice Huffman fought off criticism from religious leaders, saying Proposition 19 would help eliminate law enforcement practices that disproportionally target African Americans.
A group of former law enforcement officers also endorsed the measure, saying it would free up the courts and police to work on more serious crimes.
However, a coalition of medical marijuana advocates is coming out against the ballot initiative.
The California Cannabis Association said that Proposition 19 would inadvertently harm patients by allowing local governments to prohibit the sale and purchase of marijuana in their jurisdictions.
The group predicts many cities and counties would impose such a ban if voters approve the initiative, leaving local medical marijuana users with few options.