Journalism with a point of view is a fine thing. It's what I do. The difference is that I say I'm a liberal journalist while Fox executives and "reporters" insist they play it straight. But everyone in the US knows that my description is true. This is precisely why its fans watch it. Walk into any bar, hair salon, gym or motel lobby in the country; if the TV is tuned to Fox rather than CNN, you know that the owner or clientele or both are Republican. It's a secret - although not actually secret any more - sign of fraternity among conservatives, the way a solid red tie worn by a single urban man used to signal to other urban men that the wearer was indeed "that way".

So everyone knows, but, because of the conventions of journalistic propriety, Fox can't admit that it's a Republican outfit. It would have no credibility with politicians if it did and would be too easily dismissed as "ideological media". To get around this problem, its marketers devised what must be the most deviously ingenious pair of advertising slogans of all time: "We report, you decide" and "Fair and balanced".

And so, for a decade and more, Fox has got away with an amazing thing: it can call itself a "straight" news channel even while everyone knows it's not. It's a great little racket. Every so often, a Toto comes along and tugs at the curtain - earlier this year, for instance, the Democratic presidential aspirants agreed that they would not participate in any debates hosted by Fox because there was no point in getting up there and being asked questions merely for the purpose of providing footage that the eventual Republican nominee could use against them. But these moments have been rare.

Last week brought an event with the potential to change all that. Judith Regan, a former Fox host perhaps best known in the UK as the, um, brains behind the OJ Simpson If I Did It mediapalooza, has sued her former employer for wrongful dismissal.

So what? So this. Regan spent some portion of the dawn of the 21st century having an affair with NYC's then police commissioner, Bernard Kerik. The commissioner was recently indicted by a federal prosecutor in New York for alleged misdeeds dating from his time as a public servant. Kerik is a very close associate of presidential hopeful Rudy Giuliani - so close that Giuliani once recommended Kerik to President Bush as homeland security director.

The nomination advanced far enough for Bush to stand at Kerik's side at a press conference. But suddenly, the doors blew open and the allegations against Kerik - that he'd renovated his home with ill-gotten gains, and more distressingly that he had suspected connections to organised crime - ended his nomination quickly. Ever since then, the question has loomed over Giuliani: when did he know that the man he recommended to run America's security was alleged to have mob ties? (A now deceased investigator once suggested that he warned Giuliani, but Giuliani says he has no memory of this.)

Regan, naturally enough given her special knowledge of the man, was questioned about Kerik by federal investigators. And she now alleges that two executives of Fox News instructed her to "lie to, and withhold information from" the investigators about Kerik. Regan charges that Fox executives did this because they feared the inquiry into Kerik might singe Giuliani, whose presidential ambitions, her complaint charges, Fox has long been intent on "protecting".

Let's linger over that for a moment. Two executives of a major news organisation may have told a citizen to lie to federal investigators to protect a presidential candidate. It's a stunning charge. If proven someday, Fox will no longer be able to hide behind the fiction that it's a neutral news outfit.

In the meantime, Democrats should ratchet up their refusal to pretend that Fox bears any relationship to news. I've always felt they should just boycott the network en bloc. One can be pretty confident that if the situation were reversed - imagine a cable channel that was known as a Democratic house organ and run by, say, Bill Clinton adviser James Carville - Republicans would have done something like that long ago. I asked Nancy Pelosi, the House Democratic speaker, about this last Friday, and she just replied wanly: "I think we have to reach out to all the viewers out there."

I guess I didn't really expect her to say more on the record. But if the day ever comes that Fox is no longer allowed to have it both ways, Democrats won't have to keep playing along with the rabbit-hole fiction that Fox is a genuine news-gathering operation.

· Michael Tomasky is editor of Guardian America

michael.tomasky@theguardian.com