How did this happen? How did a fantasy president from a world of make believe come to govern a country whose power was built on hard-headed materialism? To find out, take a look at two squalid little stories which have been concluded over the past 10 days.

The first involves the broadcaster CBS. In September, its 60 Minutes programme ran an investigation into how George Bush avoided the Vietnam draft. It produced memos which appeared to show that his squadron commander in the Texas National Guard had been persuaded to "sugarcoat" his service record. The programme's allegations were immediately and convincingly refuted: Republicans were able to point to evidence suggesting the memos had been faked. Last week, following an inquiry into the programme, the producer was sacked, and three CBS executives were forced to resign.

The incident couldn't have been more helpful to Bush. Though there is no question that he managed to avoid serving in Vietnam, the collapse of CBS's story suggested that all the allegations made about his war record were false, and the issue dropped out of the news. CBS was furiously denounced by the rightwing pundits, with the result that between then and the election, hardly any broadcaster dared to criticise George Bush. Mary Mapes, the producer whom CBS fired, was the network's most effective investigative journalist: she was the person who helped bring the Abu Ghraib photos to public attention. If the memos were faked, the forger was either a moron or a very smart operator.

It's true, of course, that CBS should have taken more care. But I think it is safe to assume that if the network had instead broadcast unsustainable allegations about John Kerry, none of its executives would now be looking for work. How many people have lost their jobs, at CBS or anywhere else, for repeating bogus stories released by the Swift Boat Veterans for Truth about Kerry's record in Vietnam? How many were sacked for misreporting the Jessica Lynch affair? Or for claiming that Saddam Hussein had an active nuclear weapons programme in 2003? Or that he was buying uranium from Niger, or using mobile biological weapons labs, or had a hand in 9/11? How many people were sacked, during Clinton's presidency, for broadcasting outright lies about the Whitewater affair? The answer, in all cases, is none.

You can say what you like in the US media, as long as it helps a Republican president. But slip up once while questioning him, and you will be torn to shreds. Even the most grovelling affirmations of loyalty won't help. The presenter of 60 Minutes, Dan Rather, is the man who once told his audience" "George Bush is the president, he makes the decisions and, you know, as just one American, he wants me to line up, just tell me where." CBS is owned by the conglomerate Viacom, whose chairman told reporters: "We believe the election of a Republican administration is better for our company." But for Fox News and the shockjocks syndicated by Clear Channel, Rather's faltering attempt at investigative journalism is further evidence of "a liberal media conspiracy".

This is not the first time something like this has happened. In 1998, CNN made a programme which claimed that, during the Vietnam war, US special forces dropped sarin gas on defectors who had fled to Laos. In this case, there was plenty of evidence to support the story. But after four weeks of furious denunciations, the network's owner, Ted Turner, publicly apologised in terms you would expect to hear during a show trial in North Korea: "I'll take my shirt off and beat myself bloody on the back." CNN had erred, he said, by broadcasting the allegations when "we didn't have evidence beyond a reasonable doubt". As the website wsws.org has pointed out, it's hard to think of a single investigative story - Watergate, the My Lai massacre, Britain's arms to Iraq scandal - which could have been proved at the time by journalists "beyond a reasonable doubt". But Turner did what was demanded of him, with the result that, in media fairyland, the atrocity is now deemed not to have happened.

The other squalid little story broke three days before the CBS people were sacked. A US newspaper discovered that Armstrong Williams, a television presenter who (among other jobs) had a weekly slot on a syndicated TV show called America's Black Forum, had secretly signed a $240,000 contract with the US Department of Education. The contract required him "to regularly comment" on George Bush's education bill "during the course of his broadcasts" and to ensure that "Secretary Paige [the education secretary] and other department officials shall have the option of appearing from time to time as studio guests".

It's hard to see why the administration bothered to pay him. Williams has described as his "mentors" Lee Atwater - the man who, under Reagan's presidency, brought a new viciousness to Republican campaigning - and the segregationist senator Strom Thurmond. His broadcasting career has been dedicated to promoting extreme Republican causes and attacking civil rights campaigns.

What makes this story interesting is that the show he worked on was founded, in 1977, by the radical black activists Glen Ford and Peter Gamble, to "allow black reporters to hold politicians and activists of all persuasions accountable to black people". They sold their shares in 1980, and the programme was later bought by the Uniworld Group. With Williams's help, the new owners have reversed its politics, and turned it into a recruitment vehicle for the Republican party. Williams appears to have been taking money for doing what he was doing anyway.

These stories, in other words, are illustrations of the ways in which the US media is disciplined by corporate America. In the first case the other corporate broadcasters joined forces to punish a dissenter in their ranks. In the second case a corporation captured what was once a dissenting programme and turned it into another means of engineering conformity.

The role of the media corporations in the US is similar to that of repressive state regimes elsewhere: they decide what the public will and won't be allowed to hear, and either punish or recruit the social deviants who insist on telling a different story. The journalists they employ do what almost all journalists working under repressive regimes do: they internalise the demands of the censor, and understand, before anyone has told them, what is permissible and what is not.

So, when they are faced with a choice between a fable which helps the Republicans, and a reality which hurts them, they choose the fable. As their fantasies accumulate, the story they tell about the world veers further and further from reality. Anyone who tries to bring the people back down to earth is denounced as a traitor and a fantasist. And anyone who seeks to become president must first learn to live in fairyland.