Well, there were multiple founders of the ACLU, and some of them were communists, but not all. Here’s a list, broken down by party affiliation.
Roger Baldwin- Sociologist, pacifist, member of the IWW…if you had to label him, “syndicalist” would be the best term.
Crystal Eastman- Feminist, suffragette, editor…she was a communist.
Norman Thomas-Presbytarian minister, pacifist, 3 time presidential candidate…socialist
Jane Addams-Feminist, pioneered the “halfway house” concept, pacifist…socialist
Florence Kelley-founding member of the NAACP, lecturer on labor issues, pacifist…communist in early life, became socialist.
Lillian Ward-pacifist, social reformer…socialist
Felix Frankfurter-lawyer, professor, later, Supreme Court Justice…Democrat
Oswald G. Villard-founding member of the NAACP, pacifist, newspaper owner…socialist
Paul Kellogg-journalist, sociologist, pacifist…socialist
Clarence Darrow-lawyer…socialist
John Dewey-philosopher, psychologist, educatior…socialist
Charles Beard-historian, pacifist…socialist
Abraham Muste-minister, pacifist…socialist
Elizabeth Gurley Flynn-labor organizer, feminist…first socialist, then communist
Upton Sinclair-novelist, politician…socialist
So, while all of the founders were left of center, only 3 at most were communists. In fact, if you needed to find some trait to bind them together, it would be pacifism.