For Barry McKernan and Saavik Ford, academics who live with their baby son in New York, sitting down to the television news is a nightly ritual, as it has been for generations of families. But there the tradition ends. Instead of watching heavyweight presenters dispense news from on high, the couple switch to The Daily Show with Jon Stewart, known for delivering stories with lacerating humour and an inbuilt bullshit detector. They know Stewart will part politicians from their reputations with laser-like precision, while simultaneously rubbing the media's nose in its own deference. And, more importantly, they trust him.

'The Daily Show is probably more reliable for news than anything on TV except PBS [Public Broadcasting Service],' said McKernan, 36, who teaches astronomy at the Borough of Manhattan Community College. 'It stands apart from everything else because it unspins the news. It frankly points out how ridiculous the 24-hour news networks are - mostly gassing away by unqualified 'experts' filling the hours.'

McKernan and Ford - young, liberal, politically engaged - are typical of a section of America that has grown sceptical of what they see as mainstream television's bland, knee-jerk journalism. For them only one man is asking the right questions, and that is a 45-year-old comedian called Jon Stewart. Like a court jester, he segues into truths that solemn courtiers cannot or dare not, announcing to the startled throng that the emperor has no clothes. Ford, 30, reflected: 'The mainstream news media appear to me to be too lazy to do their jobs as well as a comedian.'

The Daily Show is satire with substance, a spoof news programme in which Stewart mercilessly punctures the headlines and skewers the powerful with cleverly edited film clips, sharp one-liners and bemused expressions. It features interviews - guests have included Victoria Beckham, Bill Clinton, Tom Cruise and Pervez Musharraf - and parodic news items from a team of roving reporters. Ridiculous stories delivered with a straight face, in the style of the fake newspaper The Onion, are combined with the bracing iconoclasticism of Michael Moore minus earnestness or ego. British viewers - who can catch it on the More4 channel four times a week or via the Comedy Central website - might be reminded of the Nineties news spoof The Day Today with Chris Morris and Steve Coogan, Angus Deayton's irreverent hosting of Have I Got News for You and Armando Iannucci's taste for the politically preposterous. Add the rottweiler instincts of John Humphrys or Jeremy Paxman, and throw in some downright silliness, and you have something approaching The Daily Show.

Broadcast on cable channel Comedy Central, it has prospered in the Bush years with steadily growing viewing figures - still below 2 million but, like the New York Times (circulation 1.1 million), punching above its weight with opinion formers. Its bite-sized chunks of laugh-out-loud smartness are also perfectly geared for internet virals and the so-called 'YouTube generation'. A blogger called Matt Tobey may have only got slightly carried away last week when he wrote: 'I wasn't alive to see Michelangelo paint the ceiling of the Sistine Chapel. I wasn't born yet when the Beatles toured. And I probably won't ever get out to see that Japanese dude eat all them hotdogs. But goddamn, seeing Jon Stewart at his absolute best running circles around cable news douchebags is almost as good.'

This year's presidential election has marked Stewart's coming of age as a cultural and political force in America. Whereas his debut as Oscars host in 2006 saw some jokes fall flat, he returned this year in triumph with timely political gags. Appearing on The Daily Show - described by Newsweek magazine as 'the coolest pit stop on television' - is now a gamble that no would-be president can afford to duck if they want to parade their 'human side' and ability to laugh at themselves. John McCain, the Republican candidate, has been on more than a dozen times over the years. Barack Obama, his Democratic rival, used The Daily Show for his last TV appearance before the primary election in Pennsylvania. Hillary Clinton - 'the first viable presidential candidate with a working uterus' - appeared on the eve of the crucial primaries in Texas and Ohio. At the Democratic national convention, Daily Show reporters found it hard to work as they were mobbed by so many fans.

It is a delicious paradox that people's search for truth has led them to fake news. Under the headline 'Is Jon Stewart the Most Trusted Man in America?', the New York Times opined recently: 'The Daily Show resonates not only because it is wickedly funny but also because its keen sense of the absurd is perfectly attuned to an era in which cognitive dissonance has become a national epidemic. Indeed, Stewart's frequent exclamation "Are you insane?!" seems a fitting refrain for a post-M*A*S*H, post-Catch-22 reality, where the surreal and outrageous have become commonplace - an era kicked off by the 2000 election standoff in Florida, rocked by 9/11 and haunted by the fallout of a costly war waged on the premise of weapons of mass destruction that did not exist.'

Showing politicians condemn themselves out of their own mouths is a classic Stewart manoeuvre. In one sequence, The Daily Show spliced excerpts from McCain's acceptance speech at the Republican convention with one by Bush before he became president. The word-for-word similarities were uncanny, promising change in Washington, a pro-life culture and the rest. It was a masterpiece of editing that required no narrative to tear down false idols, arguably more effective than anything the Democrats have thrown at McCain in recent weeks. It makes Obama's argument that McCain represents a third term of Bush more eloquently than the candidate himself has so far managed.

Stewart's admirers in the political and media classes are legion. David Remnick, editor of the New Yorker magazine, told The Observer: 'He is our most astute - and, obviously, most hilarious - press watchdog and overall political bullshit monitor. His most effective move is to cull through the tapes of all the countless banalities, hypocritical contradictions and attempted snow-jobs executed in boundless profusion on our airwaves and on political podiums. He just puts them on the air and you watch with slack-jawed amazement.'

Remnick added: 'Stewart is all the things the best stand-up comics are - quick, verbally deft - but he is also possessed of a political and moral sense. There are days when I think he is the only one keeping a sane head.'

The Daily Show also satirises American media with its cliches, hang-ups and petty obsessions. For those who accuse Rupert Murdoch's Fox News of being a partisan Republican mouthpiece, there seemed to be no antidote until The Daily Show came along. It also strikes a chord with those who feel the 24-hour news channels hype every gaffe into a hurricane and despair of US newspapers as their circulations fall and they lay off staff.

The lack of aggression in challenging the official intelligence used to justify the Iraq war, and the timidity around revelations of John Edwards's extramarital affair are just two examples of a perceived cancer eating into American journalism.

Step forward Stewart, filling the void not with new facts - he does not claim to be an investigative reporter - but forensic editing and satirical snap. Iraq was quickly branded 'Mess O'Potamia'. While mainstream TV news endlessly repeated the Pentagon's description of 'the coalition of the willing' backing US forces, Stewart was quick to dub them the 'coalition of the piddling' and point out that allies such as Morocco were sending not troops but 2,000 monkeys to set off landmines. Other media were forced to play catch-up when the show highlighted the links between Halliburton, of which Vice-President Dick Cheney was chief executive, and government wartime contracts worth millions.

Gavin Esler, the BBC's Newsnight presenter and former chief North America correspondent, describes Stewart as 'hugely influential' in the US. 'I think his success tells you a great deal about the dire state of some American network television journalism. It is often parochial, sentimental and focused on a diet of crime stories, supposedly "human interest" stories and medical "breakthroughs". It is no coincidence that the biggest cheer Sarah Palin received in her Republican convention speech was when she attacked the mainstream media.

'Stewart is highly intelligent, perceptive and persistent as an interviewer but also funny. He allows his guests to be human and is not sycophantic. The best interviews I have seen this year with Obama, Hillary Clinton and McCain have all been by Stewart. It's hardly surprising they appear on his shows - they get to demonstrate their own sense of humour, which is very appealing to voters.'

Many on the right believe the show is too liberal, in a country where that can be something of a dirty word. Stewart has admitted he is looking forward to the end of the Bush administration 'as a comedian, as a person, as a citizen, as a mammal'. But The Daily Show goes beyond party politics. When political debate in the most powerful country on Earth boils down to the semantics of 'putting lipstick on a pig', its stiffest challenge is to keep fake news more absurd than the real thing. Keep succeeding, and Jon Stewart might even swing the election.