This is what they were created to do, and I applaud them for it.
You'll find a number of (imo short-sighted) self-described liberals who burned their membership cards when the ACLU took on a case involving defending the right of a neo-Nazi group to stage a gathering in Skokie, IL. This incident was actually used by the right-wing to try to discredit the ACLU, a fact that is rich with irony.
Look up this book:
http://www.kansaspress.ku.edu/strwhe.htmlAs I said ... unpopular causes. In matters of free speech, the guiding principle of the ACLU is one of purity. The First Amendment was intended to protect unpopular speech especially, which was the fundamental basis of their reasoning in taking on the Skokie case. What's quite interesting is that those groups and individuals who are most openly opponents of the ACLU and what it does are often the first to turn to it when they find their own feet held to the fire ... again, neo-Nazis in Skokie.
As a general, wandering idea to explore, look into the usage of the term "card carrying liberal" as a pejorative. It and the the phrase "card carrying member of the ACLU" are cousins.