… is from page 335 of the 1996 Johns Hopkins University Press edition of H.L. Mencken’s 1956 collection, On Politics: A Carnival of Buncombe; specifically, it’s from Mencken’s November 2, 1936, article – published on the eve of the national election pitting incumbent president F.D.R. against challenger Alf Landon – “The Choice Tomorrow”:
For people in the mass soon grow used to anything, including even being swindled. There comes a time when the patter of the quack becomes as natural and as indubitable to their ears as the texts of Holy Writ, and when that time comes it is a dreadful job debamboozling them.