The union seeking to organize fast-food workers is adopting the unusual tactic of trying to form an alliance with disgruntled franchisees of big chains.

The Service Employees International Union said on Monday it has filed a petition with the Federal Trade Commission to investigate what the labor group claims are abusive practices on the part of major franchise companies including McDonald's Corp. and 7-Eleven Inc. The union alleges that large franchisers have failed to disclose pertinent financial information to prospective franchisees and have terminated franchises without cause, among other things.

The service employees union has been attacking the fast-food industry and other franchise-dependent sectors on a range of fronts. The union has gained attention for its push for higher wages for fast-food workers by backing a series of protests in cities around the country, and has pushed for franchisers to be treated as joint employers with its franchisees.

The union's latest tactic is unusual in that franchisees were seen as a target of its efforts so far. They own the lion's share of restaurants at McDonald's and many other big chains and generally set wages for their workers.

McDonald's said it supports and has "a strong working relationship" with its 3,100 independent franchisees in the U.S., and that the union's appeal to franchisees is ironic since the group has worked over the past two years "to disrupt operations of these same businessmen and women."

7-Eleven didn't immediately respond to requests for comment.

An FTC spokesman said the agency often receives petitions "about any number of subjects and we always take them seriously." He declined to say whether the FTC would investigate this particular petition or to explain what the process would be or how long it would take.

The union claims franchisers haven't been transparent about the financial health of their systems and that they haven't disclosed the true cost of required capital expenditures such as restaurant remodeling and equipment upgrades. Union officials on Monday didn't explicitly say what bearing those claims have on restaurant workers, but said the union's latest effort is aimed at rebalancing power between "the little guys in the economy"—including franchisees and workers—and "the big guys at the top."

Scott Courtney, the union's organizing director, said workers' pay raises—even at franchised restaurants—should come not from the franchisees but from the chains' profits. He said McDonald's should use cash for that instead of giving it to shareholders in the form of stock buybacks.

It doesn't appear that the union has built a very broad coalition of support among franchisees, however. During a conference call on Monday announcing the petition, only three current or former franchisees spoke: a 7-Eleven franchisee, a former McDonald's operator who has been lobbying for a franchisee rights bill in California, and a current McDonald's franchisee in Puerto Rico who claimed that the company has made it harder for small franchisees there.

The union is asking the FTC to gather extensive information on the practices of large franchisers and issue recommendations on how to prevent such practices.

Write to Julie Jargon at julie.jargon@wsj.com

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