In the era that you are asking about, the American Civil Liberties Union worked in two main ways to try to promote and defend civil liberties in the United States. The ACLU was, in some ways, split into two camps, each of which advocated a different approach.
One camp within the ACLU felt that education and the promotion of legislation were the best ways to push for the protection of civil liberties. They felt that change had to come from below. If they could educate people about the value of civil liberties, they would be able to get the government to change laws and to ensure that civil liberties would not be violated.
The other camp felt that the courts were the best hope for change. They supported the strategy of bringing lawsuits to prevent the government from infringing on civil liberties. This strategy was responsible for the ACLU’s most notable achievements in the time that you are asking about. For example, the group was able achieve a victory in the case of Gitlow v. New York. In that case, the Supreme Court held for the first time that states could not abridge a person’s freedom of speech any more than the federal government could. This was the first time any part of the Bill of Rights had been applied to the states.
These were the main strategies used by the ACLU during the 1920s and 1930s.