Union membership has declined sharply over the last six decades, from nearly 35 percent of the workforce in the 1950s to just 11.3 percent in 2014. To reverse this trend, the nation's largest union — the Service Employees International Union — has come up with an audacious new strategy, one that may provide a big payoff, but which also carries great financial risk. Unfortunately for the SEIU's dues-paying members, that risk is compounding while success remains elusive.

The SEIU's strategy focuses on the fast food industry, a huge pool of non-unionized service workers. It's a potentially lucrative target, worth hundreds of millions annually in dues revenue, but one that traditionally has been out of reach due to unique factors in the industry.

First, employee turnover is high. This makes winning a majority of workers time consuming and expensive at best and impossible at worst. Another hurdle is that a union wouldn't necessarily gain much from winning an election. Most fast food establishments are separately-owned, franchised businesses that provide few inroads for broader union organizing at other restaurants with the same brand name.

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The SEIU's strategy, which is designed to overcome these obstacles, has several elements. The first is to recruit large numbers of workers in the industry through rounds of nationwide protests at fast food restaurants — protests galvanized around an attention-grabbing theme like a $15-an-hour minimum wage. The second is to push wage hikes with sympathetic local governments to demonstrate policy successes. The third — and perhaps most important — is to convince the federal National Labor Relations Board to eliminate the legal separation between brand name companies and local franchise business owners — making them a seamless entity for purposes of unionizing.

The SEIU's strategy has made at least some progress. There have been several rounds of protests focused on the "Fight for $15," although just a handful of cities have seen more than minimal participation. A few local governments, notably Seattle, Los Angeles and San Francisco, have passed a $15 minimum wage. And the NLRB is poised to issue a decision in a case called Browning-Ferris that could break down the legal separation between brand names and local franchise business owners by rewriting the so-called "joint-employer" standard.

But the financial risks are mounting fast. The SEIU has spent a minimum of $30 million (likely substantially more) pursuing its strategy over the past two years, which is a hefty sum even for a union of its size. And it has little to show for it.

Protests haven't led to new union members. Moreover, the Fight for $15 contrivance the SEIU created seems to be taking on a life of its own and may no longer be just a union recruitment vehicle. This was glaringly illustrated in Los Angeles when the city council considered its $15 minimum wage. Although unions had been vocal advocates of the proposal, the Los Angeles Federation of Labor belatedly sought an exemption for unionized workplaces, arguing it was needed to protect collective bargaining. Non-union advocates of the Fight for $15 raised an uproar, and even Kendall Fells, the SEIU staffer charged with leading the fast food protests, had to concede that $15 should apply to all workers, unionized or not. And finally, the long-expected Browning-Ferris decision has not yet been issued. The SEIU must hope that whenever it comes, it enables them to quickly recruit new members without the need for lengthy organizing campaigns or additional litigation.

Because, as anyone in the business world knows, at some point there must be a return on investment or the investors, in this case the dues payers who are footing the bill, run out of patience. Only union members know how much longer they will be content watching leadership throw good money after bad. But it's a question that should raise concerns at SEIU headquarters as the bills continue to mount.

Glenn Spencer is vice president of the Workforce Freedom Initiative at the U.S. Chamber of Commerce. To learn more, visit www.workforcefreedom.com. Thinking of submitting an op-ed to the Washington Examiner? Be sure to read our guidelines on submissions.