WHEAT RIDGE, Colo.—Colorado medical marijuana dispensaries are getting a boost in political clout after they joined the United Food and Commercial Workers Union.

The U.S. Justice Department has warned that marijuana dispensaries and licensed growers in states with medical marijuana laws could face prosecution for violating federal drug and money-laundering laws, which union members are vowing to fight.

Daniel J. Rush, a national marijuana dispensary organizer for the union, said dispensary owners are at war with the U.S. Justice Department in California over the issue, and he believes the unions can help them.

“They declared war on us, we didn’t declare war on them,” Rush said.

Dispensaries in Michigan, California, and Washington state also are seeking union backing.

Dispensary representatives said Monday that they believe medical marijuana is part of the retail health care, agriculture, and food processing industry.

“Our union has over 100 years’ experience representing these very types of workers. We intend for our collective bargaining agreements to serve as a first line of regulation in the communities our members work in, and we are proud to do it,” said Kim Cordova, president of the UFCW Local 7.

The union chapter has about 25,000 members in Colorado and Wyoming representing grocery, pharmacy, agriculture workers, food processors, meat cutters, textile workers, and health care workers.

As of July, 16 states and the District of Columbia have legalized the medical use of marijuana, with programs in various phases of development.

Brian Vicente, executive director of Sensible Colorado, a medical marijuana patients’ group, said union backing sends a message that professional organizations are keeping the medical marijuana industry in place.

He said union support, including campaign and financial backing, could help get a constitutional amendment on the Colorado ballot next year that would regulate and tax recreational marijuana to raise money for schools.

Steve Ackerman of the Organic Alternatives dispensary in Fort Collins said about 200 people working for 20 licenses marijuana businesses are counting on union support to fight a measure on the Nov. 1 ballot that would ban medical marijuana businesses in the city. If approved, the 20 licensed marijuana businesses in the city would have 90 days to shut down.

So far, opponents of the ban, including dispensary owners, have raised nearly eight times more money than ban supporters.

Marijuana dispensary advocates said the medical marijuana industry employs more than 8,000 workers and contributes millions of tax dollars to the Colorado economy.

Ray Martinez, who got the initiative on the ballot prohibiting marijuana dispensaries and grow operations, said Fort Collins has become a sanctuary city for medical marijuana and making them unionized won’t make them any more legitimate.

“Unions are about making money, and that’s what marijuana stores are about. They don’t care about the patients,” Martinez said.