The quotation in the question is the version that appears in The Yale Book of Quotations (2006 edition, p. 512) and it is this version that is usually used today. The full original version, as published by H. L. Mencken, is this: "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." ('Notes On Journalism' in the Chicago Tribune, 19 September 1926).

More Info: en.m.wikiquote.org