“Flak” (anti-aircraft fire) became a widely used term during World War II. “If you’re not catching flak, you’re not over the target” (or “If you’re catching flak, you’re probably over the target”) was a military phrase.
 
The political use of “flak” (a barrage of criticism) began by at least 1968, when the New York (NY) Times reported “flak” involving New York City Mayor John Lindsay. People were described as “catching flak,” and a 1970 Tom Wolfe book was titled Radical Chic & Mau-Mauing Flak Catchers.
 
Political candidates have turned “catching flak” (intense criticism) into a positive thing, showing that they’re striking a “nerve” or a “target.” Independent president candidate Col. James “Bo” Gritz said in 1992: “When you’re catching flak, you’re on target.” Republican presidential candidate Mike Huckabee said in 2008: “The air force has a saying that says if you’re not catching flak, you’re not over the target. I’m catching the flak; I must be over the target.”
 
A similar saying is, “When you’re getting kicked in the rear, it means you’re in front.” Both sayings have been popular in business and in politics.
 
 
Merriam Webster’s Online Dictionary
Main Entry: flak
Variant(s): also flack \ˈflak\
Function: noun
Inflected Form(s): plural flak also flack
Etymology: German, from Fliegerabwehrkanonen, from Flieger flyer + Abwehr defense + Kanonen cannons
Date: 1938
1 : antiaircraft guns
2 : the bursting shells fired from flak
3 also flack : criticism, opposition
 
(Oxford English Dictionary)
flak, n.
fig. (A barrage of) abuse, adverse criticism.
1968 N.Y. Times 20 May 46 In spite of the current flak between Mayor Lindsay and..the..administrator of Boston and New Haven.., the potential for the city is unlimited.
1969 A. LURIE Real People 163 Well, all right. So why all the flak?
1972 New Yorker 16 Jan. 41 (caption) Getting much flak from Women’s Lib
 
flak-catcher colloq. (orig. U.S.), one who deals with and deflects adverse or hostile comment, questions, etc., in order to protect a person or institution from unfavourable publicity; also transf.
1970 T. WOLFE Radical Chic & Mau-Mauing Flak Catchers 110 And then it dawns on you… This man is the *flak catcher. His job is to catch the flak for the No. 1 man.
1976 Weekend Mag. (Montreal) 31 Jan. 7/4 The cynic sees regulatory agencies as flak-catchers for the government.
1984 Melody Maker 6 Oct. 14/5 They defend themselves pretty well, putting up their wittiest flak-catchers for interview.
 
11 June 1992, The Gazette (Cedar Rapids-Iowa City, IA), “Presidential hopeful Gritz stumps in C. R.” by John Kirsch, pg. 4:
“There is an old combat axiom: ‘When you’re catching flak, where are you? You’re on target,”’ Gritz countered Wednesday in a news conference at the Sheraton.
(James “Bo” Gritz—ed.)
 
Google Groups: alt.activism
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From: .(JavaScript must be enabled to view this email address) (John Covici)
Date: 11 Aug 92 5:45:4 GMT
Local: Mon, Aug 10 1992 11:45 pm
Subject: news 08/07/92
   
Independent presidential candidate Col. James “Bo” Gritz, USA ctstet (ret.) told the weekly {New Federalist} that one of his first acts as President would be to free statesman Lyndon LaRouche from prison. In an interview in the July 20 issue, Gritz said: “I believe that Lyndon LaRouche is a political prisoner. I believe that all who were imprisoned, who were part of his organization, are political prisoners…. We have an old combat saying, which I think applies in this case: `When you’re catching flak, you’re on target.’”
       
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From: apple

Date: 2000/10/25
Subject: Re: How did Harry do on “Meet The Press” ?
 
> I’m sure I’ll catch flak from the third party types for
> my assessment, but it’s not like I shy away from
> controvery in this NG… :Oo
 
If you’re catching flak it just means you are over the target.
         
Google News Archive
24 October 2003, The Argus-Press (Owosso, MI), “If I had a hammer” by Diane Carey, pg. 4, col. 3:
To those who disagree with my opinions—welcome aboard! If I’m not catching flak, I’m not hitting the target. Public discourse is the backbone of America and I’m privileged to be part of it.
       
6 March 2004, Kerrville (TX) Times, pg. 3C, col. 2:
One person gave some good advice, telling me the worst that could happen in any campaign was winning, for then the actual work would begin. “Friends become rarer and enemies accumulate,” another said. Still another told me if I was catching flak, I must be near the target.
(...)
Joe Herring Jr. is a Kerrville native who remembers several campaigns with fondness.
   
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From: .(JavaScript must be enabled to view this email address)
Date: 27 Feb 2005 11:41:47 -0800
Local: Sun, Feb 27 2005 1:41 pm
Subject: Re: Why should God have to explain himself to you bums
 
> “You know you’re over the target when you start receiving flak.”
 
You got the Bo Gritz quote wrong: 
“When you’re on target, you’re catching flak”
   
Google Books
I Fail to Miss Your Point
By Jim O’Bryon
Longwood, FL: Xulon Press
2007
Pg. 28:
If you’re not catching flak, you’re probably not over the target.
(Senator Barry Goldwater to Honorable Jack Krings, DOT&E, Pentagon, 1985)

New York (NY) Times
Republican candidates spar in South Carolina debate
By Michael Cooper and Michael Luo
Published: Tuesday, January 1, 2008
(...)
Huckabee, for his part, responded with trademark humor. “The air force has a saying that says if you’re not catching flak, you’re not over the target,” he said. “I’m catching the flak; I must be over the target.”
 
Washington Week with Gwen Ifill
Friday, January 11, 2008
GOV. MIKE HUCKABEE (R-AR): The Air Force have a saying that says that if you’re not catching flak, you’re not over the target. I’m catching the flak. I must be over the target.
   
tobefree
January 7, 2009
FreedomQuotes: “If you’re not catching flak you’re not over the target”
Filed under: FreedomQuotes, Lying • ToBeFree, People Cursing People — Jeff Fenske @ 4:14 am
 
This apparently originated from WWII bomber pilots.
They could tell when they were over the target
when they could see, hear and feel the flak.
. 
“If you’re not catching flak,
you’re not over the target.”
- Anonymous