WordNet (r) 3.0 (2006):

mudslinger
    n 1: one who spreads real or alleged scandal about another
         (usually for political advantage) [syn: muckraker,
         mudslinger]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "mudslinger":
   Thersites, backbiter, belittler, calumniator, caustic critic,
   cynic, debunker, decrier, defamer, deflater, depreciator,
   derogator, detractor, disparager, hatchet man, knocker, libeler,
   muckraker, railer, slanderer, social critic, traducer