WordNet (r) 3.0 (2006):

fellow traveler
    n 1: a communist sympathizer (but not a member of the Communist
         Party) [syn: fellow traveler, fellow traveller]
    2: a traveler who accompanies you [syn: companion, fellow
       traveler, fellow traveller]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "fellow traveler":
   Bolshevik, Bolshevist, Castroite, Communist, Communist sympathizer,
   Leninist, Maoist, Marxist, Marxist-Leninist, Red, Stalinist,
   Titoist, Trojan horse, Trotskyist, Trotskyite, avowed Communist,
   bodyguard, bolshie, cavalier, chaperon, collaborationist,
   collaborator, commie, companion, conductor, convoy, crypto, duenna,
   escort, esquire, fifth column, fifth columnist, fraternizer, guard,
   revisionist, saboteur, safe-conduct, security risk, shepherd,
   squire, subversive, swain, underground, usher