[Bahbah]

I think you pretty much have the idea.

[quidnunc]

fellow traveller n

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

Source: WordNet ® 2.0, © 2003 Princeton University

[nopardons]

It's an OLD term.Someone who wasn't a card carrying member of the American (or other) Communist Party,but was not only sympathetic to the cause,but also aided and abetted them,was called a " fellow traveler";also a "pinko"...meaning not quite "RED",or Commie.

I hope that helps. :-)

[Belisaurius]

"Useful Idiot" is an alterantive label for such people.

[RightWingAtheist]

The term is referred specifically to people who are not de facto communist party, but nonetheless openly support its goals, and the means they take in fulfilling them. I would strongly suggest that you not use the term against your debating opponent, as ad homneim comments are the lowest form of rhetorical strategy, and they'll just blast you back for being a "McCarthyite".

[Texas Eagle]

I don't know how soon "soon" is, but if you have time to buy a book called "Radical Son" by David Horowitz, you'll hear from a self-professed fellow traveller exactly what a fellow traveller is.