On 12 September this year, during the Indian summer of America's discontent, tens of thousands of rightwing protesters marched on Washington. The issues at stake were many – Obama's proposed overhaul of healthcare, high taxes, big spending, feared socialism, abortion – and the venom was extraordinary. Placards featured Obama as the Joker – in whiteface, with his mouth slashed bloodily from ear to ear, and the caption "THE DOCTOR WILL SEE YOU NOW".

It was some time around then that the White House launched a war on the Murdoch-owned Fox News Channel – or that Fox launched a war on it, depending on who you think threw the first bomb, and when. "War", in any case, was the White House's word. Communications director Anita Dunn explained that the cable network – which has more than double the viewers of its closest competitor – was "undertaking a war against Barack Obama and the White House" and that as a result "we're not going to legitimise them as a news organisation". Fox, Dunn went on to say, "often operates almost as either the research arm or the communications arm of the Republican party". (Since then, Fox's ratings have shot up, Obama has altered his strategy by giving an interview to one of its reporters, and Dunn has stepped down as communications director.)

The suggestion that Fox skews right is not new, and not especially contentious. What is new is its position in a spectacularly energised opposition movement that has taken hold in Obama's first year. George W Bush's senior adviser Karl Rove used to keep Fox in step with a Republican agenda; now the Republicans are no longer in power, Fox is beholden to nothing other than its own desire to make money. As the Republican party recovers from its defeat, Fox appears to have stepped into the breach with a conservative, at times renegade agenda. It has become the galvanising force of a rightwing protest movement the likes of which the country has never seen.

In November 1963, the American historian Richard Hofstadter gave a lecture at Oxford which became a famous essay: "The Paranoid Style in American Politics". "Although American political life has rarely been touched by the most acute varieties of class conflict," Hofstadter began, "it has served again and again as an arena for uncommonly angry minds". He coined the phrase "paranoid style" to evoke, as he put it, "qualities of heated exaggeration, suspiciousness and conspiratorial fantasy", and explained that he used the term the way an art historian might write of the baroque style or the mannerist style. He was referring not to the clinical paranoid but to the more or less normal person who speaks in this idiom of persecution. The clinical paranoid thinks the world is against him and him alone; the political paranoid believes he speaks for millions.

This is just the style of speech whose renaissance we are witnessing. In an article published in the New Yorker shortly after the September protests, Hendrik Hertzberg – a leading political commentator and former speechwriter for Jimmy Carter – pointed out that although this administration knew that overhauling the healthcare system would be difficult, what came as a surprise to them was "the predominant tone of opposition". "This sort of lunatic paranoia has long been a feature of the fringe," Hertzberg wrote. "What is different now is the evolution of a new political organism, with paranoia as its animating principle".

The protesters – who call themselves the tea party protesters, after the resistance movement that gave rise to the Boston Tea Party in 1773 – differ in crucial ways from leftwing protest movements of the past. They have the allegiance of Republicans in Congress, rather than operating in counterpoint to government. They are backed by vast sums of corporate money – in a practice known as "astroturfing" (fake grassroots), corporate sponsors funnel cash into a variety of non-profit groups so that their interests are served by what appears to be a spontaneous populist movement. And, thanks to Fox, they have a very, very loud voice.

Scattered between the protest signs on 12 September were endorsements of one of the march's instigators: "WE LOVE GLENN BECK". Fox News presenters such as Sean Hannity and Bill O'Reilly have been part of the inflammatory political landscape for some time. People invest in them: one weekend in October, five books in the New York Times bestseller list were by conservative pundits either employed by or affiliated with Fox. But Fox's newest star, its most artful misinformer, is Glenn Beck. His 5pm show on Fox now claims 3 million viewers; his syndicated morning radio show has 8 million listeners; he has several bestselling books, including a novel and a children's book; he has his own one-man touring stage show and, as of last week, conducts political rallies. (His earnings in the year leading up to June 2009 were estimated by Forbes to be around $23m, and they are set to increase.)

Until the beginning of this year, Beck – derided as "Satan's mentally challenged younger brother" by Stephen King – was at CNN. When Obama was elected, Beck had modest, reasonable things to say about him. "I think so far he's chosen wisely," he told Time magazine. "I frankly pissed off a lot of my real diehard Republicans when I said: 'He is my president. He is your president.' We must have him succeed. If he fails, we all fail." But as soon as Beck moved to Fox and Obama moved into the White House, Beck became a completely different animal – the leader, you might say, of the opposition.

His haranguing of Van Jones, Obama's special adviser for green jobs, contributed to Jones's resignation in September. His opposition to Cass Sunstein, Obama's choice for the Office of Information and Regulatory Affairs, helped to delay Sunstein's appointment by half a year (more on that later). After Obama's comment that the police acted stupidly when they wrongfully arrested the Harvard scholar Henry Louis Gates Jr, Beck called the president a racist, prompting an ad boycott. After a programme in which Beck said, with great melodramatic effect, that he was "tired", Obama appeared to respond directly. "I want everybody to know who's standing in the way of progress," he told a crowd in San Francisco, "I'm not tired". Two weeks ago Sarah Palin announced that she would consider Beck for her running mate should she bid for the presidency.

Unlike his colleagues, who are diehard conservatives, Beck is unpredictable – a rogue, as Sarah Palin might say. There's something giddy about watching him: he's insanely noxious, horribly funny and utterly compelling. He mocks himself for emphasis. "I am the most enthusiastic capitalist since Adam Smith," he said on one recent show, "If I could sell sponsorship on this chin right here, I would. It would say: 'third chin sponsored by Goodyear'." He convinces people that he really cares. "I'm just a guy who cares an awful lot about my country," he once said, tears of patriotism rolling down his cheeks. In the expression of one Beck-watcher, he has more going on in his face at any given moment than Jack Nicholson in his prime.

Beck's personal story is so exquisitely tailored to American dreams of redemption that it seems (and indeed parts of it may be) too bad to be true. He was born in 1964 into a working-class family in Washington state. His father came from a long line of bakers, and his mother was an alcoholic and drug addict who drowned in 1979. In 1977, Beck's parents had divorced, and he had landed his first radio gig (at the age of 13). Beck later said that his mother committed suicide when he was 13, though her death was reported as an accident and occurred two years later. (His wife said that the first she heard of his mother's suicide was when he told the story on the radio.)

As a child, he was a fan of Orson Welles's radio broadcasts, and – although Welles's political position was as far from Beck's eventual one as possible – Beck modelled his larger-than-life radio character on him. (His company is named Mercury Radio Arts, after Welles's.) In 1983, he married his first wife, and they had two daughters, one born with cerebral palsy. He became successful as a Top 40 morning radio host; after the Reagan administration dismantled a lot of the regulatory structure for radio in the mid-80s, Beck became a star of what was known as gonzo radio. He took to cocaine and alcohol – or, as he puts it in his book The Real America, he succumbed to the ministrations of "Dr Jack Daniels". In 1995, Beck was given six months to live: eight months later he was still an alcoholic. Eventually, he joined AA, pulled through, divorced, remarried, had two more children, and, at the behest of his disabled daughter, converted to Mormonism.

His experience of suffering and salvation has led him, he has said, to his quixotic brand of libertarianism. He is against the two-party system altogether. He doesn't want the government telling him what to do. "When did we become this country where everything is too big to fail?" he rhetorically asked the CBS TV interviewer Katie Couric, "What about the little guy?"

In his bestseller Glenn Beck's Common Sense (a reference to Thomas Paine's Common Sense, written in 1776), he addresses his reader. "I think I know who you are," he writes. "You are a person of 'strong beliefs', with a 'warm heart'. You work hard, you're not reckless with money, you're worried about what the economy means for your family. You're not a bigot, but you stopped expressing opinions on sensitive issues a long time ago because you don't want to be called a racist or a homophobe if you stand by your values and principles. You don't understand how the government can ask you to make more sacrifices just so that bankers and politicians can reap the benefit. Dear reader, Glenn Beck can help you. He will stand up with you and say: 'Don't tread on me.'"

One of Beck's campaigns featured a security system now being offered with some General Motors cars. It's called Onstar; it includes a tracking device in case your car gets stolen, and a speed limit you can impose so a thief can't get away in a high-speed chase. Beck's take on this was that since GM is now partly owned by the government, this was the government's "spy satellite", its way of knowing where you are at all times. Yet Beck positions himself as an entirely reasonable, non-conspiracy theorist. He's just like you: an ordinary guy who wants to know what's going on. He refers to others as "the nut-job fringes" and says: "the fringes are what's pulling us apart".

He also holds some surprising opinions. He has said he would have voted for Hillary Clinton over John McCain; he's in favour of gay marriage; he quotes Martin Luther King all the time. Beck is known to suffer from attention deficit hyperactivity disorder. "We all feel a wide range of emotions," Beck writes, "and as a borderline schizophrenic, I assure you my range is wider than it should be."

What does the proliferation of active and outspoken conservatives mean for America? Are they nothing but noise, or are they genuinely powerful? And if they're powerful, are they a threat to Democrats – a force that will steer the Republicans further to the right and take most of the country with them? Or are they a threat to Republicans – a movement that will accentuate the fractures in the opposition party and render it unable to govern again?

When I ask the liberal columnist Michael Tomasky whether we're looking at a loony fringe, he says: "The fringe is the mainstream. I think a key point here is that with each passing decade since Ronald Reagan, the Republican party has moved further and further to the right. In the 1980s, Ronald Reagan seemed really conservative. If a person of Ronald Reagan's position and politics were around today, these people would probably call him a sellout. I could not name you six Republicans in Congress who seem like they're prepared to negotiate in good faith on anything that's remotely controversial."

Tomasky directs me to a poll published last week. One of Beck's big targets has been an organisation called Acorn, for which Obama once worked as a lawyer and which helped him get out the vote during his presidential campaign. Republicans accused Acorn of voter fraud, and this year it has been the subject of embezzlement and other scandals, to which Fox has given a great deal of coverage. As a result, this poll suggests, a majority of Republicans thinks the election was stolen. "Only one in four Republican voters thinks Obama won the election legitimately," Tomasky concludes in amazement. "What do you do with that? It's like trying to argue with people who think that the grass is blue and the sky is green."

James Pinkerton, a long-time Fox contributor who worked for Reagan and Bush senior, believes the tea party movement represents the mainstream, not just of the Republican party, but of the country as a whole. He refers to another recent poll, conducted by Gallup in which 40% of Americans described themselves as conservatives, 36% as moderates and 20% as liberals. "Conservatives outnumber liberals two to one," Pinkerton emphasises. "The tea party movement is simply public opinion registering itself, and it would appear that they've got the bulk of the country with them." So, as for the nutty fringe, he says drily, "I'm sure in Europe you could find people who'd say America's at least 40% nutty, but from an American point of view, this is who we are."

Then I speak to David Frum, author of Comeback: Conservatism That Can Win Again and a former speechwriter for Bush (Frum is responsible for the phrase "axis of evil"). He is much more specific about Fox's relationship to the Republican party (and, by extension, its possible effect or non-effect on the Democratic party). Frum says: "If you're someone like me – a Republican who would like to govern the country again – Fox is a gigantic trap. There is no way this kind of talk is going to govern American public opinion as a whole."

He quotes a story told by the novelist Tom Wolfe (a famous Republican), about an occasion when he met the German novelist Günter Grass (who has his own complicated political history). Grass said: "The dark night of fascism is falling in the United States!" and Wolfe said: "The dark night of fascism may be falling in the United States, but it always seems to land in Europe." In other words, Frum says: "This is a highly stable political system. And the system screens out extremists. It's just that the entertainment system rewards extremists. So there's a gap. The real confusion in the Republican party right now, the real danger, is that it will appease the Palinites much more than it needs to."

Frum cites the example of Cass Sunstein, the legal scholar Obama appointed to head the Office of Information and Regulatory Affairs – in Frum's description, one of the most important jobs in the US government. Sunstein is in favour of deregulation, and a darling of, among others, David Cameron. He was endorsed by the Chamber of Commerce, by the Wall Street Journal; in short, this was a nomination people on the right were very pleased with. But, Frum explains: "Glenn Beck takes it into his head that this guy is bad news." The result is that 33 Republicans vote against confirming him to the job. "That's the Fox problem: it pushes the party into doing things that it knows it doesn't want to do – that its most important constituencies oppose, that put you on the wrong side of where the potential swing voters are. In order to appease the Fox audience, you end up being more radical than you need to be."

"Radical right, you mean?"

"I'm not sure I want to use that word. Radical angry."

Frum concludes that Republicans "have a lot to worry about" in the face of Fox and its friends. In the 90s, he says, Republicans would have used something like this healthcare bill to push through changes they wanted without leaving their fingerprints on them, and let the Democrats take the blame if they were unpopular. Now, however, these televisual carny barkers are in the way, and "it makes it impossible for your leadership to make any kind of deal".

Essentially, Frum is describing the exact thing referred to by Tomasky, whose political position is diametrically opposed: Republicans in Congress are not negotiating. But Frum doesn't attribute it to the party's right-wing drift. He says the party is subject to a powerful force outside its border. In other words, Fox is changing the face of bipartisanship – that notion by which Obama sets so much store. The network is neither a meaningless noise machine nor an arm of the Republican party but, you might say, a third party, an active splitter of views and votes. Glenn Beck, the renegade opponent of the two-party system with his unpredictable agenda and paranoid messages, is therefore not some uncontainable figure on the fringe but the most explicit spokesperson for this exploding purpose.

The White House may have got that line about Fox being a wing of the Republican party backwards: The Republican party, it seems, is now a wing of Fox.