In March 2014, Sean Caldwell went to the first Philadelphia rally for the Fight for $15, the national campaign to raise the minimum wage and to help fast-food workers — who earn one of the lowest median wages in the country — form a union for the first time. Caldwell, an Army veteran and college student, who was 35, had been working for nearly two years as a janitor on the early-morning shift at a McDonald’s in North Philadelphia. Four days a week, he cleaned frying vats, unloaded delivery trucks and mopped and restocked the restaurant. He made $8 an hour.

At the rally, Caldwell handed out fliers and held a cardboard sign that asked drivers to honk in support of “$15 and a union.” He saw himself on TV that night in news about the protest. About a week later, Caldwell arrived at work two hours late for his 6 a.m. shift, after having been with his daughter in the emergency room past midnight. He’d tried to call in to say he’d be late, but no one answered the phone and there was no way to leave a message.

In Caldwell’s experience, it wasn’t unusual for employees, including managers, to be late to work. Shortly after he got to the restaurant that morning, he was fired. A few days later, when he went back to return his uniform, he said, a manager handed him a stack of write-ups dating back months, for lateness and other minor infractions, which he had never seen before. She asked him to sign them. Caldwell refused, because he didn’t know whether they were accurate, and suspected he was being fired for his involvement with the union.

It’s against the law to fire workers for labor organizing. The Service Employees International Union, which backed the Fight for $15 and represents almost two million workers in health care and other fields, in 2014 took Caldwell’s case, along with the complaints of hundreds of other workers who thought they’d been penalized for participating in the Fight for $15, to the National Labor Relations Board for an investigation. The federal agency was created in 1935 as part of the National Labor Relations Act, the cornerstone of American labor law, which has a mandate to protect the rights of employees “to join together” to improve their wages and working conditions. In December 2014, the N.L.R.B. general counsel, an Obama appointee, grouped 60 of them for an initial trial against McDonald’s and its franchisees.