MINNEAPOLIS—They’re back, and they’re expensive.

Protests outside fast-food restaurants flared up in cities around the country this week, organized by groups with ties to prominent labor unions.

The groups organizing the protests—with names such as Citizens Action of New York and Fast Food Workers United—have used a mix of Occupy Wall Street populism and Big Labor tactics to draw attention to their cause.

But another goal seems to be drawing more members into the union.

>>> Here’s How the Price of Your Favorite Fast Food Would Change With a $15 Minimum Wage

The Service Employees International Union, or SEIU, is one of the biggest backers of the effort to raise the minimum wage to $15 per hour and to unionize fast-food workers. The union is heavily invested in the effort, having spent more than $38 million, directly and indirectly, in 2013 alone.

“Economics have driven this campaign since its inception more than two years ago,” said Ryan Williams of Worker Center Watch, a nonprofit that tracks union spending and opposes efforts to raise the minimum wage. “The SEIU has dumped millions of dollars into quasi-union worker centers to create the mirage of an organic movement.”

Worker Center Watch recently tallied the SEIU’s expenditures on fast-food protests by sorting through disclosures filed with the U.S. Department of Labor.

>>> Chart: How Much Fast-Food Prices Would Rise If Employers Paid $15 an Hour

According to its own reports, SEIU spent more than $9.1 million on directly organizing fast-food worker protests in 2013. The union organized “fast food workers organizing committees” in seven states last year to direct the protests and other activities and funneled more than $14.7 million to so-called “workers centers”—non-union groups that don’t have to follow national labor laws and, therefore, can engage in disruptive protests and other tactics unions can’t.

The union and its workers centers brought all those elements to bear Thursday, with protests held in most major American cities. In Philadelphia, Boston and elsewhere, protesters were arrested after shutting down streets and causing disruptions — events sure to draw more attention on social media and in news coverage.

Read more on Watchdog.org.