WordNet (r) 3.0 (2006):

whispering campaign
    n 1: the organized dissemination of derogatory rumors designed
         to discredit a candidate

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "whispering campaign":
   aspersion, character assassination, dirt, dirty politics,
   gossip column, imputation, innuendo, insinuation, juicy morsel,
   last-minute lie, malicious gossip, muckraking, mudslinging,
   mudslinging campaign, personal remark, personality,
   political canard, reflection, roorback, scandal, scandalmongering,
   slander, slur, sly suggestion, smear campaign, suggestion, tidbit,
   uncomplimentary remark