In the case of Baggett v. Bullitt, 1963, the U.S Supreme Court ruled that loyalty oaths required by Washington state law for its employees were unconstitutional due to their vagueness and potential infringement on First Amendment rights. The laws in question demanded public employees swear they were not "subversive persons," a term which was ambiguously defined and could be interpreted broadly enough to include lawful political activity or speech critical of the government. The court held that such ambiguity violated due process as it failed to give clear notice of what behavior would result in punishment, thereby potentially deterring constitutionally protected speech out of fear of sanctions. This decision reinforced the principle that any restrictions on free expression must be clearly defined so as not to discourage legitimate exercise of constitutional rights.

In the dissenting opinion for Baggett v. Bullitt, Justice Harlan argued that the loyalty oath required by Washington state was not unconstitutionally vague or overly broad. He contended that it is within a state's rights to require its employees to swear an oath of allegiance and uphold the constitution, as long as they are not being asked to renounce their right to lawful political activity. Furthermore, he believed that any potential vagueness in terms like "subversive" could be clarified through judicial interpretation rather than striking down the entire statute. In his view, this would prevent individuals with subversive intentions from exploiting legal protections while still preserving freedom of speech and association for law-abiding citizens.