“I do think in the media there is a tendency to describe conservatives as one of two things: stupid or evil. And those are the two categories that every conservative gets put in by Democrats and the media. A conservative is either stupid — too dumb to know the right answer — and even worse, if they actually know the right answer, then they’re evil. They want people to suffer. I suppose I feel mildly complimented in that they have recently invented a third category, which is crazy. It’s the alternative to stupid or evil. And now crazy is the third one, because it seems inconceivable that there could be Americans who believe in free-market principles and believe in the Constitution and are working to defend them.”

In a Parliamentary debate with the Conservative MP, John Pakington (May 31, 1866). Hansard, vol 183, col 1592. Pakington was referring to Footnote 3 to Chapter 7 of Mill's "Considerations on Representative Government".
Misquoted as "I never meant to say that the Conservatives are generally stupid. I meant to say that stupid people are generally Conservative. I believe that is so obviously and universally admitted a principle that I hardly think any gentleman will deny it." in "Life of John Stuart Mill" (1889) by W. L. Courtney, p. 147.
This seems to have become paraphrased as "Conservatives are not necessarily stupid, but most stupid people are conservatives." which was a variant published in Quotations for Our Time (1978), edited by Laurence J. Peter.

Talk at the University of Edinburgh, Scotland, March 22, 2005 https://www.youtube.com/watch?v=xEvIDiVheys.
Quotes 2000s, 2005

Source: Succubus Blues

R.U.R. supplement in The Saturday Review (1923)
Context: Be these people either Conservatives or Socialists, Yellows or Reds, the most important thing is — and that is the point I want to stress — that all of them are right in the plain and moral sense of the word... I ask whether it is not possible to see in the present social conflict of the world an analogous struggle between two, three, five equally serious verities and equally generous idealisms? I think it is possible, and that is the most dramatic element in modern civilization, that a human truth is opposed to another human truth no less human, ideal against ideal, positive worth against worth no less positive, instead of the struggle being as we are so often told, one between noble truth and vile selfish error.

Same with the Texas LCRs, for whom I’ve been signing books for years.
Dear Mainstream Media Reporter Who Wasted My Time (February 2, 2011) http://www.humanevents.com/2011/02/09/dear-mainstream-media-reporter-who-wasted-my-time/.
2011

"The Man with the Mustache"

Quotes from interviews, Sydney Morning Herald interview (2003)
Context: You can make fun with Saddam Hussein jokes … but you can't make fun of, say, the concentration camps. I think my target was not so much evil, but benign stupidity people doing stupid things without realising or, instead, thinking they were doing good.

Original: Una delle peggiori categorie di individui è quella che vuole sempre aver ragione sapendo di non averla.
Source: prevale.net

The News Quiz series 72, episode 1 (BBC Radio 4, 24 September 2010).