Morris Leopold Ernst was an American lawyer and prominent attorney for the American Civil Liberties Union (ACLU). In public life, he defended and asserted the rights of Americans to privacy and freedom from censorship, playing a significant role in challenging and overcoming the banning of certain works of literature and in asserting the right of media employees to organize labor unions. He also promoted an anti-communist stance within the ACLU itself, and was a member of the President's Committee on Civil Rights.

The American Civil Liberties Union (ACLU) is an American nonprofit human rights organization founded in 1920. The organization strives "to defend and preserve the individual rights and liberties guaranteed to every person in this country by the Constitution and laws of the United States." The ACLU works through litigation and lobbying and has more than 1,800,000 members as of July 2018, with an annual budget over $300 million. ACLU affiliates are active in all 50 states, Washington, D.C., and Puerto Rico. The ACLU provides legal assistance in cases where it considers civil liberties at risk. Legal support from the ACLU can take the form of direct legal representation or preparation of amicus curiae briefs expressing legal arguments when another law firm is already providing representation.