WordNet (r) 3.0 (2006):

centrist
    adj 1: supporting or pursuing a course of action that is neither
           liberal nor conservative [syn: centrist, middle-of-
           the-road]
    n 1: a person who takes a position in the political center [syn:
         centrist, middle of the roader, moderate,
         moderationist]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "centrist":
   Laodicean, apathetic, center, compromiser, conservative, even,
   fifty-fifty, half-and-half, impartial, independent, indifferent,
   middle-of-the-road, middle-of-the-roader, midway, moderantist,
   moderate, moderationist, moderatist, neuter, neutral, nonaligned,
   noncommitted, nonpartisan, on the fence, passive, third-force,
   third-world, uncommitted, uninvolved