The U.S. Supreme Court case Baldwin, Commissioner of Agriculture & Markets, et al. v. G.A.F Seelig Inc., 1934 involved a dispute over the constitutionality of a New York law that prohibited the sale of milk bought outside the state unless it met in-state price regulations. The plaintiff, G.A.F Seelig Inc., was an interstate milk distributor who argued that this law violated the Commerce Clause by imposing undue restrictions on interstate commerce and trade practices. The defendants were representatives from New York's Department of Agriculture and Markets who enforced these laws to protect local dairy farmers from out-of-state competition selling at lower prices. In its decision, led by Justice Benjamin Cardozo, the Supreme Court ruled in favor of Seelig Inc., finding that while states have broad powers to regulate their own economies for public welfare purposes under their police power rights; they cannot do so in ways that discriminate against or unduly burden interstate commerce as per Dormant Commerce Clause doctrine which prohibits state legislation discriminating against interstate or international commerce. This landmark ruling established important precedents regarding federalism and economic regulation - affirming both free market principles within our national economy and limitations on states' regulatory powers when they interfere with such principles.

In the dissenting opinion for Baldwin, Commissioner of Agriculture & Markets, et al. v. G.A.F Seelig, Inc., Justice McReynolds argued that New York's law did not violate the Commerce Clause because it was designed to protect public health and welfare rather than restrict trade between states. He believed that a state should have the right to regulate its own internal affairs without interference from other states or federal government unless there is clear evidence of discrimination against interstate commerce. He also emphasized that this case involved milk - an essential commodity with unique sanitary concerns - which justified stricter regulations compared to other goods in commerce. Furthermore, he pointed out that New York's law applied equally to all milk dealers within the state regardless of whether they were local or out-of-state businesses; hence it did not discriminate against interstate commerce but aimed at maintaining fair competition and protecting consumers' interests.