Take "leftist." As a pair with "rightist," it had a long history as a purely descriptive term before the McCarthyites adopted it as a label for Communist sympathizers and subversive organizations.
Just before the 1952 election, Senator Joseph R. McCarthy accused Adlai Stevenson of being unfit for the presidency because of his association with "leftists" like Arthur M. Schlesinger Jr., who had defended the right of Communists to teach in universities. (Schlesinger had qualified his position by adding, "so long as they do not disqualify themselves by intellectual distortions in the classroom," a clause McCarthy ignored.)
That same year, Americans for Democratic Action indignantly denied charges that it was a leftist group, pointing out that it had worked at "purging the American liberal movement of individuals with loyalties to Communism."
http://www.nytimes.com/2003/08/17/weekinreview/17NUNB.html?pagewanted=1