H.L. Mencken, 18 September 1926, The Baltimore Sun

"No one in this world, so far as I know — and I have searched 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.  The mistake that is made always runs the other way.  Because the plain people are able to speak and understand, and even, in many cases, to read and write, it is assumed that they have ideas in their heads, and an appetite for more.  This assumption is a folly."

As the passage appeared, syndicated to The Chicago Tribune the following day.

Link