The U.S. Supreme Court case Florida et al. v. United States et al., in 1933, revolved around the issue of whether or not a federal court could enjoin state officials from enforcing an unconstitutional state statute that interfered with interstate commerce and postal services. The State of Florida had enacted legislation regulating milk distribution which was challenged by the Federal Government as it affected shipments across states lines and also impacted mail delivery due to its effect on transportation schedules. In this landmark decision, the Supreme Court ruled that federal courts do have authority to prevent enforcement of such statutes when they infringe upon constitutional rights or interfere with powers vested in Congress under the Constitution - specifically those related to regulation of interstate commerce and operation of postal services. This ruling underscored supremacy clause principles, affirming that state laws cannot supersede federal law where constitutionally granted powers are concerned; thus reinforcing balance between states' rights versus national interests within our system's dual sovereignty framework.

In the dissenting opinion for Florida et al. v. United States et al., Justice McReynolds expressed his disagreement with the majority's decision to uphold a federal law that regulated milk prices in Washington D.C, arguing it was beyond Congress' constitutional authority. He contended that such regulation of local affairs should be left to individual states and not controlled by federal legislation, as per the Tenth Amendment which reserves powers not delegated to the U.S by Constitution or prohibited by it to states or people respectively. According to him, this case represented an overreach of congressional power into areas traditionally governed by state law - specifically agriculture and dairy production - thereby undermining principles of federalism enshrined in American constitution.