One who supports the aims or philosophies of a political group without joining it. A “fellow traveler” is usually one who sympathizes with communist doctrines but is not a member of the Communist party. The term was used disparagingly in the 1950s to describe people accused of being communists.
A person who sympathizes with a cause or doctrine, without openly identifying himself with it : These people are not hipsters, they are fellow travelers
[1930s+; said to have been coined by Leon Trotsky in Russian as sputnik and translated into English as ''fellow traveler'']