No one in this world, so far as I know — and I have researched the records for years, and employed agents to help me — has ever lost money by underestimating the intelligence of the great masses of the plain people. Nor has anyone ever lost public office thereby. (Chicago Tribune, Sept. 19, 1926)
This is another case of a quote that mutated over the years into a pithier, more rhythmic form. An interesting divergence is that Mencken never explicitly said the American public.