“Never underestimate the power of stupid people in large groups” has been often credited to “unknown” or to comedian George Carlin (1937-2008), but the saying was used before the start of Carlin’s comedy career. In June 1959, it was credited to Allen Sparks; in July 1959, it was credited to Jack Jarvis.
 
The saying has been placed on many gift items, such as T-shirts and bumper stickers.
 
 
Never Underestimate the Power of Stupid People in Large Groups
The Stupid People Bookstore
 
Good Reads
“Never underestimate the power of stupid people in large groups.”
— George Carlin
   
19 June 1959, Rockford (IL) Register-Republic, “Off the Beat” by Byron Baxter, pg. 22B:
What’s the Motto
Nobody has filed suit against OTB yet for stealing and running, in toto, last month’s newsletter of Let’s Have Better Mottoes association.
(...)
“Never understand the power of stupid people in large groups.”—Allen Sparks.
 
9 July 1959, Omaha (NE) Evening World-Herald, pg. 26, col. 2:
Never underestimate the power of stupid people in large groups.—Jack Jarvis.
     
2 July 1963, Emporia (KS) Gazette, “Smiles,” pg. 4, col. 1:
Never underestimate the power of stupid people in large groups, admonishes the Wichita Eagle.

29 January 1966, Arizona Republic (Phoenix, AZ), pg. 71, col. 1:
Who Wants Excellence?
Sign Saying “Never Underestimate the Power
Of Stupid People in Large Groups” Not Amusing
By LAWRENCE LAURENT
Washington Post Service
LOS ANGELES—There is a sign hanging in the office of the Screen Producers’ Guild that proclaims: “Never Underestimate the Power of Stupid People in Large Groups.”

The sign was put there by Lou Greenspan, a gracious gentleman who is director of the guild and editor of the Screen Producer’s monthly magazine. He finds the sign “simply amusing” but as one wanders about this city built on pre-fabricated daydreams there is at least a suspicion that the sign proclaims a way of life.
 
25 June 1995, Aberdeen (SD) Daily News, pg. 15: 
Never underestimate the power of stupid people in large groups.
 
5 August 1998, Washington (DC) Post, “With Drum Roll, the Best T-Shirts of 1998” by Bob Levey, pg. D20:
“Never Underestimate the Power of Stupid People in Large Groups”
   
Google Books
Out of Order
By Bonnie MacDougal
New York, NY: Ballantine Books
1999
Pg. 29:
It reminded her of the second law of politics: never underestimate the power of stupid people in large groups
 
Google Books
Ryan’s Rock:
A retired minister search for spiritual sanity
By David L. Buche
Xlibris Corp,
2009
Pg. 61:
I remember a sign that hangs in my son’s office: “Never Underestimate the Power Of Stupid People In Large Groups”. I think it might be more appropriate to read, “Never Underestimate the Power Of Ignorant People In Large Groups”.
 
Google Books
If You Meet George Herbert on the Road, Kill Him!:
Radically Rethinking Priestly Ministry
By Justin Lewis-Anthony
London: Mowbray
2010
Pg. 195:
A visitor to the rectory door once gave me, for reasons never adequately explained, a fridge magnet which said, ‘Beware the power of stupid people in large groups’ (illustrated with a fetching portrait of Ronald Reagan in his leading man days); this is true and wise.
 
Google Books
Neverisms:
A Quotation Lover’s Guide to Things You Should Never Do, Never Say, or Never Forget
By Mardy Grothe
New York, NY: Harpercollins
2011
Pg. ?: 
Never underestimate the power of very stupid people in large groups.
JOHN KENNETH GALBRAITH
 
Hartford (CT) Courant
All-American Sport: Hating Congress, Politicians
Even John Adams Didn’t Like It
August 07, 2011|By Jim Shea
(...)
“Never underestimate the power of stupid people in large groups.” (Unknown)