In 1937, a branch of the American Civil Liberties Union had been established in Puerto Rico following the United States investigation of the Ponce massacre.

On May 21, 1948, a bill was introduced before the Puerto Rican Senate which would restrain the rights of the independence and Nationalist movements on the archipelago.

He used the hábeas corpus action, in order to expedite the release of Puerto Rican political prisoners, and to question the constitutionality of the Gag Law.

He brought a habeas corpus action before the U.S. Supreme Court, on behalf of Enrique Ayoroa Abreu, who had been arrested following the Ponce massacre.

Amadeo Semidey and other attorneys also defended 15 members of the Puerto Rican Nationalist Party who were charged with violating this law.

This included Concurrent Resolution 1, which created the Civil Rights Commission in both the Senate and the Camera of Representatives of Puerto Rico.

U.S. President Lyndon Baines Johnson gave Amadeo Semidey the presidential pen, with which he signed the statute.