An old political joke runs:
 
“What’s the difference between a cactus and a caucus? The cactus has all the pricks on the outside.”
 
Lyndon B. Johnson (1908-1973), a Democratic senator and newly elected vice president from Texas, lost his bid to lead the Democratic caucus in 1961 and allegedly made the remark. Mo Udall (1922-1998), a Democratic congressman from Arizona, allegedly made the remark in January 1971, after he lost a vote for Democratic caucus chairman. Many other politicians have also told the joke—even politicians in Canada and Australia.
 
   
Google Books
Democratic Review
Volume 1, Issues 2-6
1975 (The Google Books date might be incorrect—ed.)
Pg. 23:
Udall said after the latter defeat that he had been assured by a majority of the Democratic Caucus that he could count on their support, but he lost in a secret ballot. This prompted him to tell reporters. “You know the difference between a cactus and a caucus? A cactus has the pricks on the outside.”
 
7 December 1978, Register-Republic (Rockford, IL), pg. A6, col. 3:
Now this here caucus term produces nothing but a big belly laugh for me, because only a few months ago I ran across the only good explanation of a caucus, and that was from former president Lyndon B. Johnson. When LBJ was senator from Texas in 1949 he tried for the minority whip post in the Democratic caucus. He told the press he had enough pledged votes from his colleagues to win. But when the vote was taken, he was far short. Asked by a newsman for the cause of his defeat, LBJ said, “It all lies in the difference between a caucus and a cactus, and the real difference is that all the offending painful protrusions on a cactus are on the outside.”
(Letter from Francis J. Carrino, Sterling—ed.)
 
Google Books
Lyndon, an Oral Biography
By Merle Miller
New York, NY: Putnam
1980
Pg. 276:
On leaving the caucus that day, he was widely reported to have remarked, “I now know the difference between a caucus and a cactus. In a cactus all the pricks are on the outside. “
 
Google New Archive
17 July 1983, Sydney (Australia) Morning Herald, pg. 134, col. 3:
Cactus power
A FAVOURITE but telling political joke doing the rounds of Canberra concerns the difference between caucus and a cactus?
 
The answer: With a cactus, the prickles are on the outside.
 
29 April 1989, Toronto (Ontario) Star, “The tribes’ weekly gathering Caucus: family feuds with no hold barred” by Val Sears, pg. D4:
But Dief came up with the classic putdown: “What is the difference between a caucus and a cactus?” he once asked reporters. “The cactus has all the pricks on the outside.”
 
20 August 1994, The Capital Times (Madison, WI), pg. 16A, col. 3:
“Harvey Duebolm, a beloved member of the Assembly with an earthy way of cutting to the quick, often said, ‘The difference between a caucus and a cactus is that a cactus has the pricks on the outside.’”
   
Salon
SATURDAY, MAR 24, 2001 04:00 AM CDT
The education of John McCain
Stabbed in the back by his congressional colleagues, the nation’s campaign finance crusader should take his battle to the people.
BY ARIANNA HUFFINGTON
(...)
One of McCain’s favorite lines is from his mentor, the late Rep. Morris Udall, who used to say that the difference between a cactus and a congressional caucus is that in Congress “the pricks are on the inside.” And if you turn on C-Span this week, you’ll see them in full bloom.
   
Des Moines (IA) Register
Cain’s unfunny ‘jokes’ make it easy to imagine harassment
KATHIE OBRADOVICH 4:00 AM, Nov 3, 2011
(...)
Humor is like crack to politicians. Making voters laugh has got to be a rush, and it’s one of the fastest ways to gain their favor. It’s especially effective when it seems a little bit edgy. Anybody who’s researched caucus history has chuckled at this old gem from Mo Udall, who ran unsuccessfully for the Democratic nomination in 1976: “I have learned the difference between a cactus and a caucus. On a cactus, the pricks are on the outside.”
 
Google Books
The 2,548 Wittiest Things Anybody Ever Said
By Robert Byrne
New York, NY: Touchstone (Simon & Schuster
2012
2,206
The difference between a caucus and a cactus is that at a cactus the pricks are on the outside.
—Morris Udall (1922-1998)
 
Google Books
The Passage of Power:
THe Years of Lyndon Johnson
By Robert A. Caro
New York, NY: Alfred A. Knopf
2012
Pg. 169:
“I now know the difference between a caucus and a cactus.“ he told Baker. “In a cactus. all the pricks are on the outside.”
 
The Mark Levin Show
January 2, 2013
On Wednesday’s Mark Levin Show: Former Congressman J.D. Hayworth fills in for Mark. (...) Also, when discussing exactly what is going on in a caucus, J.D. jokes - what’s the difference between a cactus and a caucus? In a cactus, all the pr*cks are on the outside.