Conservative politicians have complained for many years about the liberal bias of the media.

In his book "What Liberal Media" (2003)_3., Eric Alterman finds the media to be, "on the whole, far more conservative than liberal, though it is possible to find evidence for both views. The fact that conservatives howl so much louder and more effectively than liberals is one significant reason that big media is always on its guard for "liberal" bias but gives conservative bias a free pass."

A 2004 study (see below), however, shows a liberal bias.

Henry Luce, who founded Time (1923), Fortune (1930), Life (1936) and Sports Illustrated (1954) magazines, believed in Christianity, big business, the Republican Party, anticommunism, internationalism and civil rights. Many of Luce's staff did not share his belief and as Luce wisecracked, "God damn Republicans can't write." His critics maintained that Time reflected his personal leanings. He believed that objective reporting was impossible and encouraged his editors to express his own views in their articles, which were unsigned. See: Henry Luce at American Masters at PBS and Answers.com

A December 2004 study by Tim Groseclose and Jeff Milyo funded by UCLA, the University of Missouri, Stanford University, and the University of Chicago, shows that media outlets cite left-leaning think tanks more often than right-wing ones.

They start with Americans for Democratic Action (ADA), a liberal lobbying group, ratings of the voting records of Senators and Representatives. They looked at the ratings of politicians who cited any of 200 prominent think tanks positively and then looked at how various media sources quoted the same sources.
In the 2002 session, for example Ted Kennedy received an ADA score of 100 and Phil Gramm a score of 0. Politicians who cited the Heritage Foundation and Family Research Council had an average ADA score of 20, while those who cited The Economic Policy Institute and Consumer Federation of America had an average score of 81.
They converted scores to the 1999 House scale to give centrist members of Congress a score of about 50.

Various media scored as follows:

Conservative

 4.7 Tom Delay (R.-Tex.)
10.3 Bill Frist (R.-Tenn.)
12.7 John McCain (R.-Ariz.)
16.1 Average Republican   
35.4  Washington Times
21.5 Nathan Deal (D-Ga.)
39.7  Fox News' Special Report with Brit Hume
40.6 House Median
48.0 Sam Nunn (D-Ga.)
51.3 Arlen Specter (R-Pa.)
55.8  Newshour with Jim Lehrer      
56.0  CNN NewsNight with Aaron Brown
56.1  ABC Good Morning America
58.2 Senate Median
60.4  Drudge Report         
61.0  ABC World News Tonight
61.6  NBC Nightly News      
63.4  USA Today             
64.0  NBC Today Show        
65.4  Time Magazine         
65.8  U.S. News and World Report
66.3  NPR Morning Edition
66.3  Newsweek           
66.6  CBS Early Show     
66.6  Washington Post    
68.2  Constance Morella (R-Md.)
70.0  LA Times           
73.7  CBS Evening News   
73.7  New York Times 
84.3 Average Democrat 
85.1  Wall Street Journal
87.6 John Kerry (D.-Mass.)
88.8 Ted Kennedy (D.-Mass.)
99.6 Maxine Waters (D.-Calif.) 
Liberal

"The press is no substitute for institutions. It is like the beam of a searchlight that moves restlessly about bringing one episode and then another out of darkness into vision. Men cannot do the work of the world by this light alone. They cannot govern society by episodes, incidents, eruptions. It is only when they work by a steady light of their own that the press, when it is turned upon them, reveals a situation intelligible enough for a popular decision."

- Walter Lippman (1889-1974)

On a "Are the Media Biased?" an Oct. 26, 2004, Justice Talking Program on NPR the question was asked: "If there is a liberal bias why?".
One answer was: "The national press is concentrated in Washington DC, New York and Los Angeles, and the press is just reporting the attitudes of the culture they are in."

References:
1. Efron, E. (1971) The News Twisters. Los Angeles: Nash Publishing.
2. "Coloring the News: How Political Correctness Has Corrupted American Journalism ", 2003, W. McGowan
3. "What Liberal Media", 2003, Eric Alterman

Web Links:

• Media Research Center
• Fairness and Accuracy in Reporting
• Can The Media Be So Liberal? The Economics Of Media Bias, Daniel Sutter at The CATO Institute
• People for the American Way
• Federal Communications Commission
• Southern Poverty Law Center
• Surprise! Fox News is Fair and Balanced! at MarginalRevolution.com
• Illiberal Conservative Media
• >American Conservitive Union (ACU) Ratings of legislators