“I don’t believe the liberal media” was a slogan of the National Conservative Foundation in 1985. Later, the slogan was used by the Media Research Center (a conservative media research group) and its mediawatch website, NewsBusters. “I don’t believe the liberal media” (or “Don’t believe the liberal media”) has been printed on many items, such as signs, bumper stickers and T-shirts.
   
The slogan is usually promoted most often during election years, especially at Republican conventions.
 
   
Wikipedia: Media Research Center
The Media Research Center (MRC) is a politically conservative content analysis organization based in Reston, Virginia, founded in 1987 by activist L. Brent Bozell III. Its stated mission is to “prove—through sound scientific research—that liberal bias in the media does exist and undermines traditional American values” and to neutralize the perceived liberal bias of the mainstream media.
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NewsBusters
In the summer of 2005, Media Research Center launched the NewsBusters, a website “dedicated to exposing & combating liberal media bias,” in cooperation with Matthew Sheffield, a conservative blogger involved in the CBS Killian documents story. NewsBusters is styled as a rapid-response blog site that contains posts by MRC editors to selected stories in mass media.
       
1 July 1985, Richmond (VA) Times-Dispatch, “News bias battle begins,” pg. B-10, col. 2:
WASHINGTON (UPI)—A conservative group said yesterday it will spend at least $250,000—and perhaps as much as $1 million—in an advertising campaign to convince people that the major U.S> news organizations have a liberal bias.
 
Television ads sponsored by the National Conservative Foundation urged people to call a toll-free number “if you want to help fight liberal bias.” Callers receive a free bumper sticker saying “I Don’t Believe the Liberal Media” and a pitch for contributions to finance the effort.
 
Google Books
Mimesis in Contemporary Theory/ 2,
Mimesis, Semiosis and Power
Edited by Ronald Bogue.
Philadelphia, PA: Benjamin
1991    
Pg. 175:
Instead of (dis)information, a new generation of media-sophisticates (“I don’t believe the liberal media” proclaims the latest batch of bumper-stickers) craves, not knowledge, but amusement.
 
25 November 1992, The Times (Trenton, NJ), “Now the media are displeasing Democrats” by Otis Pike, pg. A10, col. 5:
At the Republican convention in Houston one of the most popular souvenirs was a T-shirt that read, “Don’t Believe the Liberal Media.” Republicans bought them because they believed the message. Media folk bought them because they believed the message was funny.
 
Google Groups: soc.bi
Who asked for a copy of my manifesto?
Elf Sternberg
4/26/93
I Don’t Believe The Liberal Media
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I honestly believe that what we saw on television, on NBC, CBS, ABC, and CNN, as well as other networks, was the Homosexual Soft Sell.  In an attempt to make gays and lesbians look like “Normal people who just love different people,” bisexuals, transgenderists and sadomasochists were quietly placed in the editorial closet.
     
Google Groups: alt.barney.dinosaur.die.die.die
politically Incorrect
Late for Dinner
2/3/96
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The Right Stickers
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S32   I don’t Believe The Liberal Media
 
Google Groups: alt.callahans
RANT: Liberal Media
Sxcc
7/17/98
Warin looks disgusted.  “I saw a bumper sticker today.”
He snaps his fingers (okay, smart guy, what do *you* call a grizzly’s digits?) and points to the X-Window behind and slightly above himself.  It displays a sport-utility vehicle whose details are mostly fuzzy.  The notable exception is the offending sticker, which reads, “I don’t believe the liberal media,” where the word “liberal” is in red rather than black and is enlarged.
 
Twitter 
Dana
‏@sparkey999w
@newsbusters giving away free ‘I Don’t Believe the Liberal Media’ bumper stickers..Get yours here! http://bit.ly/cv1sgx #tcot #ocra
1:53 PM - 16 May 2010
 
Tell The Universe
I Don’t Believe the Liberal Media
Posted on August 21, 2012 by Jim
I have seen this bumper sticker a couple times now. I did a little research about where it’s coming from.
 
“Mark Levin promoted NewsBusters and its parent organization, the Media Research Center, on his radio show today. “Everyone knows the media have a very strong left-wing bias,” Levin noted.” This guy has a nationally syndicated radio show on the conservative stations – the ones that carry Rush, Hannity, etc.
 
Twitter
Jon Atkinson
‏@JontheAct
I just passed a Prius w/ a “I don’t believe the liberal media” bumper sticker. The irony, it hurt. It hurt so much.
9:19 PM - 27 Jun 2014