So why does almost any discussion of the media default to its alleged liberal tilt?
The answer is: Years of conditioning that go back, at least, to Spiro Agnew, Richard Nixon's media-bashing vice president. Or so says Alterman, who aims to overturn what he sees as a bum rap.
Alterman, 43, writes the media column for The Nation and a Web log for MSNBC.com. He's a liberal. And he's clearly fed up with the widespread assumption he challenges in his new book, "What Liberal Media? The Truth About Bias and the News."
"I think that even most journalists genuinely believe that the media are liberal," says Alterman during an interview at his Manhattan apartment. "They tend to bend over backwards to give conservatives more than a fair shake on most issues. They feel pinned in, to a considerable degree, by the perception of bias.
"In my book I wanted to say: `No! You're bowing to a false god!'
"I'm not trying to convince the media to be liberal," he insists. "I'd just like to even up the sides a little."
With his book, Alterman is hoping to embolden media professionals. But he also sounds a wake-up call to media consumers.
"I have a sense that there are loads of liberals out there who have been going crazy because they keep hearing about the liberal media," says Alterman. "And yet, more often than not, the media are dominated by conservatives."
His head count of conservative commentators speaks for itself (especially since Phil Donahue, one of the few liberals Alterman could come up with, has since been sent back into retirement).
But Alterman's analysis of media owners turned up no right-wing conspiracy — just a practical policy of courting the largest possible audience while averting controversies that could hurt the bottom line.
"There's an enormous conservative movement in this country, and no liberal movement," says Alterman. By favoring conservatives over the far less organized or vocal opposition, media owners are simply following the path of least resistance.
To make sure the political right maintains the upper hand, it ritually blasts the media with charges that even some leading conservatives, in unguarded moments, pooh-pooh.
Alterman quotes James Baker, Pat Buchanan and William Kristol dismissing the notion of a liberal media bias.
And from Rich Bond, former Republican Party chairman, he hears the strategy behind conservatives' campaign of "left"-baiting.
"If you watch any great coach," Bond is quoted as saying, "what they try to do is `work the refs.' Maybe the ref will cut you a little slack on the next one."
As a case history of how the media cut Republicans slack at the Democrats' expense, Alterman exhaustively revisits the 2000 presidential race.
"I knew (the media) were distorting Gore's record and ignoring Bush's deceptions," he says. But when he took another look at that coverage while researching his book, "I couldn't believe how bad it was. I don't like Gore that much, but when I saw how the media mistreated him, I became quite sympathetic."
"What Liberal Media?" arrives with the United States at war against Saddam Hussein, and Alterman sees his book's thesis newly validated by the coverage.
U.S. news outlets, he says, "feel a need to prove their patriotism to conservatives, to counteract the false blame leveled at journalism for `losing the Vietnam War.'
"Instead of the straight-forward scrutiny you expect from a good doctor when you go in for a checkup," Iraq War coverage reflects a different ambition from many journalists. "They want to be as positive as they possibly can: It's the right war at the right time at the right place."
In the front row of that bully pulpit is Fox News Channel, which has always positioned itself as the news source for people who don't trust news sources.
Thus far in Iraq, says Alterman, the flag-waving Fox "makes no distinction between its journalists and the fighting forces," a mindset that got correspondent Geraldo Rivera in trouble this week when military officials accused him of airing unauthorized information about troop movements.
Also this week, Baghdad-based Peter Arnett was dismissed by NBC and "National Geographic Explorer" for giving an unauthorized interview to Iraqi state television in which he said the U.S.-led war effort had been a failure.
"I think that was a very stupid thing to do," Alterman says. "But clearly what Geraldo is accused of is a far greater offense: People could die because of what he did. Yet all the anger is directed at Arnett."
It's just the latest assault by conservatives, says Alterman. His book blows the whistle on their long-standing war plan.