WordNet (r) 3.0 (2006):

character assassination
    n 1: an attack intended to ruin someone's reputation [syn:
         character assassination, assassination, blackwash]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "character assassination":
   attack, backbiting, backstabbing, belittlement, blackening,
   calumny, defamation, defamation of character, defilement,
   denigration, depreciation, dirt, dirty politics, disparagement,
   gossip column, juicy morsel, last-minute lie, malicious defamation,
   malicious gossip, muckraking, mudslinging, mudslinging campaign,
   name-calling, political canard, revilement, roorback, scandal,
   scandalmongering, slander, smear, smear campaign, smear word,
   tidbit, vilification, whispering campaign