In Garner v. Board of Public Works of Los Angeles, 341 U.S. 716 (1951), the Supreme Court upheld the constitutionality of a loyalty oath and an accompanying affidavit required of City of Los Angeles employees. 

Loyalty oath examined employees' membership in subversive organizations

The oath in question required that employees affirm that they did not then “advise, advocate or teach” or belong to organizations that taught the forceful overthrow of the U.S. government and that they had not for the last five years. Some employees charged the oath violated several constitutional provisions, including First Amendment protections of freedom of speech and association.

Constitutionality of oath upheld

Because the city had adopted a similar ordinance that applied five years previously, Justice Tom C. Clark, joined by four other justices, denied that the law was an ex post facto, or retroactive, criminal law. He argued that it was not a bill of attainder—a legislative, rather than a judicial punishment— because it related to professional standards of employment rather than being chiefly punitive in nature. Clark further saved the oath from impermissibly impinging on freedom of speech and association by reading it so that it only applied to employees who knew of subversive activites of their organizations.

Four justices wrote separate dissents.

This Garner ruling resembled contemporary decisions in Adler v. Board of Education (1952) and Wieman v. Updegraff (1952) in upholding oaths only when it is clear that employees understand that they apply only to those who know their groups advocate violent overthrow of the government.

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