Kevin Drum refers to the 16% of American adults who "speculate that secretly planted explosives, not burning passenger jets, were the real reason the massive twin towers of the World Trade Center collapsed" as a "fringe group". It seems to me that 16% is rather larger than a fringe.
Ross Perot garnered less than 19% of the national vote in 1992, Firefox has 15% of the internet browser market in the U.S., and Apple laptops represent 12% of America's portable computer market. I don't think I'd describe Perot, Firefox, or Apple as "fringe" elements in their respective spheres. I'd say instead that they represent small, but still significant, minorities.