Complex sentence: At the end of the 1940s, White joined the Progressive Party, of which she was an active member, was involved in the Communism movement, and was a supporter of the 1948 presidential campaign of Henry A. Wallace.
Split into simpler sentences: At the end of the 1940s, White joined the Progressive Party, of which she was an active member. She was involved in the Communism movement, and was a supporter of the 1948 presidential campaign of Henry A. Wallace.