What the Fallacy?!
No True Scotsman
The Definition
Making an appeal to purity as a way to dismiss relevant criticisms of a political party or movement.
The Explanation
When a member of a political group does something bad or unpopular, leaders will sometimes claim that person wasn't a 'true' member, avoiding accountability.
Political Example
When the conservative mayor raised taxes, commentators argued that no 'true conservative' would ever do such a thing, effectively trying to write him out of the movement.