The U.S. Supreme Court case International Association of Machinists et al. v. Gonzales in 1957 revolved around the issue of whether a union could legally expel a member for crossing a picket line and continuing to work during an authorized strike, under California law which protected employees' rights to engage or refrain from collective activities such as strikes. The plaintiff, Gonzales, was expelled by his union (International Association of Machinists) after he crossed the picket line during an authorized strike and continued working; he then sued the union for reinstatement and damages in state court citing violation of his right under California law. The Supreme Court held that federal labor policy preempted state law on this matter - meaning federal laws took precedence over conflicting state laws - thus allowing unions to enforce discipline on their members according to their constitution and bylaws without interference from states’ regulations protecting individual workers' rights against collective action.

In the dissenting opinion for International Association of Machinists et al. v. Gonzales, Justice Brennan disagreed with the majority's interpretation of Section 2, Fourth of the Railway Labor Act (RLA). He argued that it was not Congress' intent to limit union membership rules and regulations solely to those explicitly listed in the statute. Instead, he believed that Congress intended for unions to have broad discretion in setting their own internal rules and procedures as long as they did not conflict with federal law or policy. In this case, Justice Brennan felt that there was no clear evidence showing a violation of any specific provision within RLA by excluding non-union members from voting on collective bargaining agreements; hence such exclusion should be considered lawful under RLA provisions allowing unions considerable latitude in determining their own affairs.