Fast-food workers will stage another day of protests in more than 150 U.S. cities on Thursday as they seek a doubling in hourly pay to $15 and the right to form a union.

Organizers expect their biggest protest to date, with fast-food workers from McDonald's, Burger King, Wendy's and KFC restaurants expected to walk off the job. They will be joined by home-care workers.

The union-backed actions, which started in New York and other major cities, have steadily gathered steam since 2012 and helped to spur a national debate about the federal minimum wage, which has been stuck at $7.25 since 2009.

Protesters, many of whom are adults clocking 40 hours or more per week at around that wage, say they cannot survive on such pay and are pressing for $15 per hour - above the roughly $11 hourly wage that experts say is the poverty threshold for a family of four.

McDonald's Corp, Burger King Worldwide Inc and other major fast-food chains do not own most of their U.S. restaurants and leave pay decisions to their franchisees, who generally say that paying their employees more will be detrimental to their businesses.

The International Franchise Association (IFA) in a statement charged that the protests are a union smoke-screen.

Unions, led by the Service Employees International Union (SEIU), are hiding "behind an altruistic plea for higher wages when what they really want is a shortcut to refill their steadily dwindling membership ranks and coffers," IFA said.

The protesters and their supporters make a similar charge, saying that the big fast-food chains are using their restaurant ownership structure to shield themselves from the actions of their franchisees and to prevent workers from organizing.

They cheered in July, when the general counsel of the National Labor Relations Board announced that McDonald's, not just its franchisees, can be liable for alleged labor law violations.

If the five-member, Democrat-controlled NLRB board agrees with the general counsel, McDonald's and other companies that depend on franchisees and contractors could be exposed to new legal risks.