Paul Krugman in an op-ed piece ("Tidings Of Comfort") in today's NYT:
In the past, there was a general understanding, a sort of implicit clause in the rules of American politics, that major parties would at least pretend to distance themselves from irrational extremists. But those rules are no longer operative. No, Virginia, at this point there is no sanity clause.
A Christmas pun, based on the famous "Yes, Virginia, there is a Santa Claus" editorial. (The Wikipedia page linked to here has a list of popular culture references to the line.) The "sanity clause" version goes back to the Marx Brothers movie A Night at the Opera (1935). The scene (as described here):
Groucho is trying to get Chico to sign a contract for an appearance of an opera singer he represents. They agree to disagree on practically every clause in the contract, ripping them out, until only the sanity clause remains.
Chico: Hey, wait, wait. What does this say here? This thing here.
Groucho: Oh, that? Oh, that's the usual clause. That's in every contract. That just says uh, it says uh, "If any of the parties participating in this contract is shown not to be in their right mind, the entire agreement is automatically nullified."
Chico: Well, I don't know…
Groucho: It's all right, that's, that's in every contract. That's, that's what they call a 'sanity clause'.
Chico: Ha ha ha ha ha! You can't fool me! There ain't no Sanity Clause!
("There Ain't No Sanity Clause" served, in 1980, as the title of a Christmas single by The Damned.)