In the 1970 case of Amalgamated Association of Street, Electric Railway & Motor Coach Employees of America et al. v. Lockridge, the U.S Supreme Court ruled that federal labor law preempted a state's right-to-work laws in certain circumstances. The court held that an employee could be fired for not paying union dues even if they were working in a "right to work" state where such actions are typically prohibited by law. This decision was based on the fact that federal labor law allows unions and employers to agree upon contracts requiring all employees to pay union dues as a condition of employment (union security agreements). Therefore, when there is conflict between these federal provisions and state right-to-work laws, the former prevails.

In the dissenting opinion for Amalgamated Association of Street, Electric Railway & Motor Coach Employees of America et al. v. Lockridge, Justice Brennan argued that the majority's decision to invalidate a union shop agreement under Section 301 was incorrect and inconsistent with previous rulings. He contended that federal labor law should not preempt state laws unless there is an explicit conflict between them or if Congress has clearly intended it so. In this case, he believed neither condition was met as Michigan's anti-discrimination law did not interfere with national labor policy but rather complemented it by providing additional protection against racial discrimination in employment contracts beyond what federal law offered. Furthermore, he criticized the majority for overstepping their judicial role by making policy decisions better left to legislative bodies such as determining whether states can provide greater protections than those provided federally.