Real Dictionary

lunatic fringe
singular noun: If you refer to a group of people as the lunatic fringe, you mean that they are very extreme in their opinions or behaviour.

lunatic fringe in American English
members on the periphery of any group, esp. political, social, or religious, who hold extreme or fanatical views
that minority considered foolishly extremist, fanatic, etc. in a political, social, religious, or other movement

lunatic fringe in British English
noun: the members of a society or group who adopt or support views regarded as extreme or fanatical