whispering campaign 

"A method of persuasion in which damaging rumors or innuendo are spread about the target, while the source of the rumors seeks to avoid being detected while spreading them (for example, a political campaign might distribute anonymous flyers attacking the other candidate). It is generally considered unethical in open societies, particularly in matters of public policy. The speed and anonymity of communication made possible by modern technologies like the Internet has increased public awareness of whispering campaigns and their ability to succeed. This phenomenon has also led to the failure of whispering campaigns, as those seeking to prevent them are able to publicize their existence much more readily than in the past. Whispering campaigns are defended in some circles as an efficient mechanism for underdogs to attack the powerful while lacking resources."

Source: Wikipedia.org

"Tonight you are a whispering campaign" -Panic! at the Disco

The whispering campaign ruined his chances of election.

whispering campaign by Tia Tragedy December 15, 2008