Samuel Dickstein was a Democratic Congressman from New York for over 20 years. In 1934, his concern about pro-Nazi activity in the U.S. led him to help establish what eventually became the House Un-American Activities Committee, which of course moved from investigating fascists and Nazis to investigating Communists.

He is also apparently the only Congressman known to have spied for the Soviet Union while a member of Congress.

Thanks to Prof. Gary Alan Fine (Northwestern University), whose talk pointed me to this story.