fellow traveler: someone who sympathizes with the aims of a particular group or movement, without being an active participant; a Communist sympathizer
Also spelled fellow-traveller, this term may have been introduced by Leon Trotsky, who discussed "literary fellow-travellers" in his 1924 book Literature and Revolution. Trotsky used the Russian equivalent to describe Russian writers who supported the revolution's aims, but did not write specifically Communist literature.
The term soon broadened to encompass sympathizers with other causes. The Nation for October 24, 1936 describes it this way: "The new phenomenon is the fellow-traveler. The term has a Russian background and means someone who does not accept all your aims but has enough in common with you to accompany you in a comradely fashion part of the way." A similar term for a leftist sympathizer, parlor pink, arose around the same time.
Fellow traveler became a widespread political insult during the late 1940s and early 1950s. During this period, Senator Joseph McCarthy of Wisconsin began his campaign to root out members of the government who might potentially hold communist tendencies. As anti-communist fervor reached a peak around the country, anyone holding political views that could be even vaguely described as socialist was accused of being, if not an actual member of the Communist Party, at least a fellow traveler. The term is still used to describe not only communist sympathizers, but those in tune with any political ideology. The Christian Science Monitor for November 5, 2009, for example, contains the line: "He is not at all an ideological fellow traveler of conservative Doug Hoffman, the candidate in New York's 23rd Congressional District . . ."