So this is how it work for a lone partisan actor in the NSA:
1. Eve, the malicious analyst, wants to tank Herman Cain's campaign. She decides to look for dirt on him.
2. Using XKEYSCORE or another database, she inputs queries to collect his email traffic, and enters his public telephone number for call records. She requests chained results so she can get traffic and call records from his associates as well.
3. Some results show up in her inbox. Before she can read them, her auditor comes to her desk to ask her why she input a U.S. person's name, a likely U.S. person's email address, and a U.S. telephone number that hasn't been annotated as belonging to a foreigner in her justification (which she left blank). Without any good justification, she is escorted from the building and has her clearance revoked. Game over. She is eaten by a grue.
She is lucky she didn't target a congressman, because she'd be in violation of policy that requires all congressman targeting to go through DIRNSA.
Basically, the only way for this to work would be to get the analyst, auditor, supervisor, and IG to be all radicals in the same political party. And they all have no problem with losing their clearances if they're found out.