The Adair v. United States case in 1907 was a landmark decision by the U.S. Supreme Court that challenged the constitutionality of certain aspects of labor law, specifically those related to union membership and employment contracts. William Adair, a railroad official, had fired an employee for being a member of a labor union - an action which violated the Erdman Act prohibiting discrimination against employees based on their union membership. The court ruled in favor of Adair with Justice Harlan arguing that this provision within the Erdman Act infringed upon employers' freedom to contract under the Fifth Amendment's due process clause and thus was unconstitutional. This ruling effectively made "yellow-dog" contracts (agreements where workers promise not to join unions) legal until they were later outlawed by Congress through legislation such as Norris-LaGuardia Act in 1932.

In the dissenting opinion for Adair v. United States, Justice Harlan argued that Congress had the power to regulate railroad companies' labor practices because these businesses were engaged in interstate commerce. He believed that this case was not about an individual's right to contract but rather a question of whether or not Congress could prevent employers from discriminating against employees who chose to join labor unions. According to him, such discrimination threatened public safety by potentially inciting strikes and other forms of unrest among railway workers which would disrupt interstate commerce. Therefore, he concluded that it fell within Congress’s authority under the Commerce Clause of the Constitution to prohibit such discriminatory practices.