lunatic fringe: someone whose zealousness in support of a cause goes beyond rational limits
Lunatic, which first appeared in the thirteenth century as a term for a mentally ill person, derives from luna, the Latin word for moon. At the time, people believed that the phases of the moon influenced attacks of madness, with the full moon inducing the worst attacks. Since the mid-twentieth century, lunatic is no longer used literally as a medical term. Instead it's applied to people whose ideas or behavior strike the speaker as irrational.
Lunatic fringe first appeared in 1913, when lunatic could still denote someone with mental illness. Theodore Roosevelt may have coined the term. In a 1913 review of the New York City Armory's current exhibit of modernist art, he wrote, "We have to face the fact that there is apt to be a lunatic fringe among the votaries of any forward movement." Roosevelt was referring to Cubists, Futurists, and the like, but people have more typically used the term to describe those on the extremes of social or political movements. Roosevelt's distant cousin, Franklin D. Roosevelt, used it this way during a 1944 speech, attacking "labor baiters, bigots, . . . silver shirts and those on the lunatic fringe . . ." These days lunatic fringe is still a popular way to refer to people ranged along the edges of the political spectrum, either to the right or left, depending on the speaker's point of view.