The Fallacy of Guilt by Association:

The fallacy of trying to refute or condemn someone's standpoint, arguments or actions by evoking the negative ethos of those with whom the speaker is identified or of a group, party, religion or race to which he or she belongs or was once associated with. A form of Ad Hominem Argument, e.g., "Don't listen to her. She's a Republican so you can't trust anything she says,"