“Never argue with a man whose job depends on not being convinced”

MSNBC Hardball host Chris Matthews frequently uses this quote that Matthews attributes to H. L. Mencken (1880-1956): “Never argue with a man whose job depends on not being convinced.” The saying means that you’re not going to win an argument with a person who’s determined not to have an open mind.
 
The alleged Mencken source of the quotation has not been found. Upton Sinclair (1878-1968) wrote in a 1935 book, “It is difficult to get a man to understand something, when his salary depends upon his not understanding it!”
   
 
Wikiquote: Upton Sinclair
Upton Beall Sinclair, Jr. (September 20, 1878 – November 25, 1968) was a prolific American author who wrote in many genres, often advocating Socialist views, and achieved considerable popularity in the first half of the twentieth century.
 
Quotes
I used to say to our audiences: “It is difficult to get a man to understand something, when his salary depends upon his not understanding it!”
. I, Candidate for Governor: And How I Got Licked (1935), ISBN 0-520-08198-6; repr. University of California Press, 1994, p. 109.
       
Wikipedia: H. L. Mencken
Henry Louis “H. L.” Mencken (September 12, 1880 – January 29, 1956), was an American journalist, essayist, magazine editor, satirist, acerbic critic of American life and culture, and a student of American English. Mencken, known as the “Sage of Baltimore”, is regarded as one of the most influential American writers and prose stylists of the first half of the 20th century.
 
Mencken is known for writing The American Language, a multi-volume study of how the English language is spoken in the United States, and for his satirical reporting on the Scopes trial, which he named the “Monkey” trial. In addition to his literary accomplishments, Mencken was known for his controversial ideas. An opponent of World War II and democracy, Mencken wrote a huge number of articles about current events, books, music, prominent politicians, pseudo-intellectuals, temperance and uplifters. He notably attacked ignorance, intolerance, frauds, fundamentalist Christianity, osteopathy, and chiropractic.
 
Google Books
Hardball:
How politics is played—told by one who knows the game
By Christopher Matthews
New York, NY: Simon & Schuster
1999
Pg. 88:
As H, L. Mencken once warned, “Never argue with a man whose job depends on not being convinced.”
 
Google News Archive
31 December 2000, Gadsden (AL) Times, “Sen. Pat Moynihan’s career, 1976-2000” by Chris Matthews, pg. A4, col. 4: 
The street smarts to spot these realities early has carried a certain price. “Never argue with a man,” a cabdriver once quoted H. L. Mencken to me, “whose job depends on not being convinced.”
 
MSNBC TV 
‘Countdown with Keith Olbermann’ for Feb. 2
Read the transcript to the 8 p.m. ET show
updated 2/3/2005 11:41:31 AM ET 2005-02-03T16:41:31
(...)
MATTHEWS: (...) HL Mencken once said, “Never argue with a man whose job depends on not being convinced.”