The Collaborative International Dictionary of English v.0.48:

left-of-center \left-of-center\ adj.
   same as left-wing.

   Syn: leftist, left-wing.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

left-of-center
    adj 1: believing in or supporting tenets of the political left
           [syn: leftist, left-of-center, left-wing]