character assassination

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The malicious denunciation or slandering of another person, especially as part of an effort to ruin the reputation of a public figure.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic a malicious verbal assault designed to damage the reputation of a public figure

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an attack intended to ruin someone's reputation