The U.S. Supreme Court case Public Service Commission et al. v. Brashear Freight Lines, Inc., et al., 1938, revolved around the issue of whether a state could regulate interstate commerce in relation to motor carriers operating within its borders without violating the Commerce Clause of the Constitution. The Public Service Commission (PSC) of Missouri sought to enforce regulations on Brashear Freight Lines, an interstate trucking company that operated within and outside Missouri's boundaries. However, Brashear argued that such regulation was unconstitutional as it interfered with their ability to conduct business across state lines freely. The Supreme Court ruled in favor of PSC stating that states have authority over highways and can therefore impose reasonable regulations on companies using them for commercial purposes even if they are involved in interstate commerce - provided these rules do not discriminate against or unduly burden interstate commerce. This decision affirmed states' rights to regulate local aspects of national businesses while also maintaining federal supremacy over broader issues related to trade between states.

The dissenting opinion in the case of Public Service Commission et al. v. Brashear Freight Lines, Inc., et al., argued that the majority's decision to allow interstate trucking companies to bypass state regulations was a misinterpretation of the Federal Motor Carrier Act and an overreach of federal power. The dissenters believed that states should retain their authority to regulate local aspects of interstate commerce when it does not interfere with national uniformity or efficiency. They contended that allowing states this control would protect them from economic harm caused by unregulated competition and ensure fair treatment for all businesses operating within their borders. Furthermore, they expressed concern about potential negative impacts on public safety due to lack of oversight if state regulatory powers were diminished.