“We have a change in the structure of work that is then compounded by a falling level of what is viewed as acceptable in the workplace in terms of how you treat people and how you regard the law,” Mr. Weil said.

His agency has uncovered nearly $1 billion in illegally unpaid wages since 2010. He noted that the victimized workers were disproportionately immigrants.

Guadalupe Salazar, a cashier at a McDonald’s in Oakland, complained that her paychecks repeatedly missed a few hours of work time and overtime pay. Frustrated about this, she has joined one of seven lawsuits against McDonald’s and several of its franchise operators, asserting that workers were cheated out of overtime, had hours erased from timecards and had to work off the clock.

“Basically every time that I worked overtime, it didn’t show up in my paycheck,” Ms. Salazar said. “This is time that I would rather be with my family, and they just take it away.”

Business advocates see a hidden agenda in these lawsuits. For example, the lawsuit against Schneider — which owns a gigantic warehouse here that serves Walmart exclusively — coincides with unions pressuring Walmart to raise wages. The lawyers and labor groups behind the lawsuit have sought to hold Walmart jointly liable in the case.

Walmart says that it seeks to ensure that its contractors comply with all laws, and that it was not responsible for Schneider’s employment practices. Schneider said it “manages its operations with integrity,” noting that it had hired various subcontractors to oversee the loading and unloading crews.

Business groups note that the lawsuits against McDonald’s have been coordinated with the fast-food workers’ movement demanding a $15 wage. “This is a classic special-interest campaign by labor unions,” said Stephen J. Caldeira, president of the International Franchise Association. In legal papers, McDonald’s denied any liability in Ms. Salazar’s case, and the Oakland franchisee insisted that Ms. Salazar had failed to establish illegal actions by the restaurant.