A “bedwetting liberal” (or “bed wetting liberal”) is a derogatory term, similar to “liberal pansy,” to describe a person weak in the bladder and (presumably) the brain. Humorist Lewis Grizzard (1946-1994) used the “bed-wetting liberal” term by at least 1988 and might have popularized it.
 
The political use of the “bedwetter” epithet in Congress appears to date to 1978.
       
   
Wikipedia: Lewis Grizzard
Lewis McDonald Grizzard, Jr. (October 20, 1946 - March 20, 1994) was an American writer and humorist, known for his Southern demeanor and commentary on the American South. Although he spent his early career as a newspaper sports writer and editor, becoming the sports editor of the Atlanta Journal at age 23, he is much better known for his humorous newspaper columns in the Atlanta Journal-Constitution. He was also a popular stand-up comedian & lecturer.
 
Grizzard also published a total of twenty-five books, including collections of his columns (e.g. Chili Dawgs Always Bark at Night), expanded versions of his stand-up comedy routines (I Haven’t Understood Anything Since 1962), and the autobiographical If I Ever Get Back to Georgia, I’m Gonna Nail My Feet to the Ground. Although much of his comedy discussed the South and Grizzard’s personal and professional lives, it was also a commentary on issues prevalent throughout America, including relationships between men and women (e.g. If Love Were Oil, I’d Be About a Quart Low), politics, and health, especially heart health. Grizzard was also the stepbrother of the Southern humorist Ludlow Porch.
 
Urban Dictionary
bedwetting liberal
Bedwetting Liberal is a derogatory term used to describe people of a political leaning. The term bedwetting is used to denote a hand-wringing, scared, worrisome, pansy attitude, afraid of their own shadows. The term liberal denotes the love of spending, social programs, handholding and acceptance of such things as illegal immigration and allowance of the same to have all of the social benefits of being in the United States. Bedwetting Liberals are generally terrified of Republicans and conservatives. They shriek at the thought of combat operations or allowing the police to do their jobs. They are quick to point fingers, cry to the media and ACLU, and snivel about attempts to maintain US heritage because it might “offend” somebody.
Despite the presence of Moses and the 10 Commandments in the structure of the US Supreme Court and other national buildings, the presence of “In God We Trust” on our currency, or the cross in the L.A. County Seal as signs of heritage, the Bedwetting Liberals are so terrified it might “offend” a Muslim or other religion, they have enlisted the ACLU to have all of this eradicated from our history.
by Patriotically Loyal Oct 8, 2006
 
21 August 1978, Newsweek, “Son of the Tax Revolt” by Peter Goldman with Henry W. Hubbard and Thomas M. DeFrank, pg. 20:
This news was received with odd docility by Kemp-Roth’s Republican promoters, who had promised an up-or-down vote and at moments dared dream they might win it. The louder cries of pain came from some edgy young liberal Democrats—“bedwetters,” in the instant coinage of the Hill—pushing for an income-tax credit to cushion the coming social-security-tax rise. Some marched on O’Neill threatening to sabotage his whole scenario—a revolt that melted on the spot when the bill’s sponsor, Rep. Richard A. Gephardt of Missouri, announced: “I can’t go along with that.”
     
Google Books
Modern Legal Glossary
By Kenneth R Redden and Enid Veron
Charlottesville, VA: Michie Co.
1980
Pg. 45:
BED WETTERS. Slang term for young liberal Congressional Democrats who panic easily when things don’t go their way.
 
Google News Archive
25 November 1988, Spartanburg (SC) Herald-Journal, “TV tabloids enter new age of sleaze” by Lewis Grizzard, pg. B2, col. 5:
Thursday, Pat Buchanan and some whiny, bed-wetting liberals.
 
Google News Archive
19 April 1989, Gainesville (FL) Sun, “Can you say ‘Won’t you just go away, NRA’?” by Lewis Grizzard, pg. 5D, col. 2:
“It is our constitutional right to have any sort of gun we want, and no bed-wetting liberals are going to take away that right.
 
“Can you say, ‘Bed-wetting liberals’? Excellent.”
 
Google Books
A heapin’ helping of true Grizzard:
Down home again with Lewis Grizzard
By Lewis Grizzard
New York, NY: Galahad Books
1991
Pg. 87:
Older people would have looked at them and instead of saying, “You godless, bed-wetting, pinko, Commie, nasty, long-haired hippies,” they might have said,...
   
Google Books
Current biography
By H.W. Wilson Company.
Bronx, NY: H.W. Wilson Co.,
1992
Pg. 80:
“They’re not gonna accuse him [Reagan] of being some bed-wetting liberal, no way can they do that,” Brady said, as quoted in the New York Daily News (March 30, 1991).
 
12 September 1992, Virginian-Pilot (Hampton Roads, VA), “Buying into family values proves to be a satisfying deal” by Lawrence Maddry, pg. D1:
Do you want your country taken over by nymphomaniacs, media elitists and leftwing, bedwetting liberal, broccoli eaters?
     
Google Books
I haven’t understood anything since 1962, and other nekkid truths
By Lewis Grizzard
New York, NY: Ballantine Books
1994, ©1992
Pg. 160:
This doesn’t mean I don’t think just because a person is gay/black/yellow/orange/purple/a communist/a bedwetting liberal /anything, he or she should be turned away from a restaurant, a hotel, or a public conveyance.
   
Google Groups: misc.invest.stocks
Newsgroups: misc.invest.stocks
From: .(JavaScript must be enabled to view this email address) (Jeff Bentley)
Date: 14 Dec 1993 08:14:52 -0800
Local: Tues, Dec 14 1993 11:14 am
Subject: Re: China related stocks are soaring!
 
David, say what!!! Take the liberal bed wetting attitude and hit the road. I’m sure there is some nice eco-company you can invest in.
 
19 September 1994, Chicago (IL) Tribune, “‘The Real World’ now a real hit” by Sid Smith, Tempo, pg. 1:
There’s Pam, a mature Asian American Harvard grad now in medical school; Judd a self-described bedwetting liberal…
   
Facebook
Fred Brill Lewis Grizzard is the Best Southern Writer of All Time:
My favorite Grizzardism is “bedwetting communist homosexual” but you just can’t say that anymore ...
October 21, 2009 at 5:31pm
   
Weasel Zippers
Assploding Chutzpah! Bed Wetting Liberal Bill Maher Says Tea Partiers Are a Bunch of Bed Wetters…
C’mon Bill, everyone with half a brain knows the left is scared of its own shadow hence the term, bed wetting liberals…
Posted by ZIP on Tuesday, September 14, 2010, at 11:39 AM