The lunatic fringe
Definition 1 of 1
Pro Tip 1/3Negative Connotation
The phrase often suggests disapproval and may be seen as dismissive towards the group it describes. 
Politicians sometimes distance themselves from the lunatic fringe of their parties.
Pro Tip 2/3Small Group
Emphasizes that the term refers specifically to a minority within a larger entity. 
Despite its noise, the lunatic fringe only represents a tiny fraction of the movement.
Pro Tip 3/3Perception Context
Understanding the phrase often depends on who uses it, as it can reflect their viewpoint on what is extreme. 
To traditional economists, those promoting cryptocurrency belong to the lunatic fringe.