When there is more than one side in a debate, an argument can be weakened by someone who is seen as stupid or a joker if they are vocal about it. This invalidates your beliefs because if crazy person also holds the same opinion and may also be misrepresenting facts, it must be untrue, and people will dismiss your factual claims.
What is this phenomenon called?
Also what is the person called who does this? I should mention there is a specific term, and it's on the tip of my tongue but I can't quite remember!