FORTUNE -- When thousands of fast-food workers form picket lines Thursday in 120 cities across the country, the purple and yellow placards of the Service Employees International Union will likely poke out from the crowd.
The largest union in the country has made it clear that it isn't out to recruit new members; it's there to fight for the survival of the working class. But by linking arms with employees of McDonalds (MCD), Wendy's, and Burger King (BKW) as they fight for a $15 minimum wage, the SEIU is taking a stand in another battle -- the effort to stay relevant.
SEIU president Mary Kay Henry, for her part, explains the union's support of the protests in this way: the wages and benefits of its members are under pressure from the growing number of jobs that offer poor pay, no benefits, and no guaranteed hours. Members' employers are asking "'why should you get a wage increase when the unemployment is at X percent and people are willing to work for minimum wage?"' Henry told Fortune on Thursday. "So, as long as that dynamic exists, we're going to continue to see a shrinking middle class because not many workers are standing up against multinational corporations that are employing low wages."
For the SEIU, a champion of the labor movement, participating in the protests is "the right thing to do," says Kate Bronfenbrenner, director of labor education research at Cornell University's Industrial and Labor Relations School.
But there's more than goodwill at play. Unionizing fast-food workers is not an SEIU priority at the moment, says Henry. That's probably for the best. The fast-food industry is thought to be nearly impossible to organize because of its franchise structure and high employee turnover. (Henry characterized those factors as "excuses" and "smokescreens" promulgated by business leaders.)
While the SEIU doesn't admit it, ("our profile is not our concern," Henry says), the fast-food worker movement, which started with small strikes in New York and Chicago a year ago, is a way for the SEIU to retain relevance in an era when labor unions are struggling to keep their clout.
A report by the Bureau of Labor Statistics from earlier this year said that union membership numbers fell by 400,000 last year, to 14.3 million, even as the nation's workforce grew by 2.4 million. Membership is at 11.3% of the workforce, the lowest level in 97 years. Right-to-work laws have trimmed the power of unions in some states. And court decisions over the past decades have – with few exceptions – favored employers' free speech and property rights over those of labor, says John Logan, professor of labor and employment studies at San Francisco State University.
"In most cases," Logan says, "any employer who has the resources and stomach for a fight will be able to prevail against employees' effort to unionize under the official National Labor Relations Board system."
When an employer goes beyond the law, for instance, by firing an employee for engaging in union activity, the penalty is to simply reinstate the worker and award back pay, an action that can take months -- if not years -- to enforce. "In the meantime, it sends a chilling message to other employees," says Logan.
As a result, workers participated in just 1,549 elections to vote for union representation last year, down from 2,675 a decade earlier, according to the NLRB.
In an era in which over half of jobs added in the U.S. since April 2010 have been in low-paying industries, even successful union campaigns barely make a dent in addressing the growing number of workers that don't make enough to survive.
The SEIU is finding channels to exert influence outside of traditional membership campaigns even though -- in contrast to organized labor as a whole -- its membership is growing. Through movements like the fast-food strikes, the SEIU "has been at the forefront of experimenting with innovative organizing tactics," says Logan. And laws recently passed in states like California and New Jersey to raise the minimum wage are valid measures of its success, says Bronfenbrenner.
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