a fellow traveler

Someone who identifies with or is sympathetic to the aims or ideology of a political movement or organization, but is not a formal or full member of it. Used especially in the 1950s in reference to those suspected of being communist sympathizers. In my grandfather's day, if someone accused you of being a fellow traveler, it was often to derail your career completely. Despite having a mark against him as a "fellow traveler," he still managed to remain at the Hollywood elite. Many people's political careers were ruined when they were incorrectly labeled "fellow travelers."

a fellow traveller

A fellow traveller is someone who supports the aims of an organization but is not a member of it. Note: `Traveller' is spelled `traveler' in American English. Although something of a critical fellow traveller, Sampson was very interested in the party.