It's not easy being a labor union in the United States. Last year, just 11.3 percent of workers were in a union, the lowest level since the 1930s. And there aren't a whole lot of opportunities for growth popping up nowadays.

Walter Reuther would be proud. (Ted S. Warren/AP )

Though here's one fascinating exception: pot. Molly Redden has a great piece in The New Republic about how the United Food and Commercial Workers (UFCW) have been trying to organize workers in California's burgeoning medical-marijuana industry.

The only problem? Perennial crackdowns by the federal government keep crimping those efforts:

The UFCW had been unionizing marijuana workers since 2010, when it organized the Lee-owned businesses that were now blocked off by fluttering caution tape. At the height of the UFCW’s efforts, more than 2,000 cannabis workers, most of who worked behind the counter selling medicinal marijuana in Western states, had signed a union contract. But federal raids have sent that figure nose-diving to 500 or so workers today. With the raid on Oaksterdam, not only had the federal government shuttered the most reputable marijuana business in the state; they had shuttered the largest union shop, of some 100 UFCW workers, in the entire cannabis industry.

One notable tidbit from Redden's piece — the UFCW has put its organizing muscle behind many of the state-level initiatives to legalize pot:

The UFCW has been an unseen force in nearly every big push to pass marijuana-friendly laws and ordinances in Western states like California and Colorado. But federal crackdowns on pot retailers remain a constant bugbear, threatening to dry up one of the UFCW’s best streams for new membership.

Reuters followed up with a story of its own, claiming that a full-fledged legalized cannabis industry in the United States could create "hundreds of thousands" of potential union workers. The Atlantic's Jordan Weissmann is skeptical of those numbers, pointing out that even the most optimistic projections for pot still envision an industry smaller than pharmacies and drug stores, which have just 77,000 workers.