The right dominates the latest web medium, allowing it to vent spleen and not be challenged. By James Crabtree

Why have Americans started to vilify the Guardian? Why does the actor John Malkovich want to kill the Independent foreign correspondent Robert Fisk? And why is the Princeton economics professor Paul Krugman writing with a new-found attention to detail? Answer: Fisk, Krugman and the Guardian are all victims of the latest web-publishing phenomenon: blogging.

Blogs are becoming the medium of choice for politically attuned members of the digital generation. Like talk radio, they are dominated by the political right. Why has the left ceded this potentially influential medium without a fight?

A weblog, or "blog", is like an internet diary in which users (bloggers) "log" and comment on interesting websites. They have grown astonishingly, from nothing to more than a quarter of a million in roughly a year. The New York technology commentator Clay Shirky dubs bloggers "trainspotters for the 21st century", obsessives who pass websites around the net, creating miniature information ecosystems in their wake.

The American right moved into the medium with speed. Two bloggers in particular have astonishing influence: the journalist Andrew Sullivan, with his eponymous site; and a formerly obscure Tennessee law professor called Glenn Reynolds, who runs InstaPundit. There are no equivalents on the left; indeed, there are precious few left-wing blogs at all. Both Reynolds and Sullivan are libertarian, rather than conservative. And both despise the Guardian.

As Wyeth Ruthven, publisher of a rare centre-left American blog, says: "No one here had even heard of the Guardian until Sullivan began his personal jihad." In a country with no recognisable left of its own, bloggers have made a British newspaper the pantomime villain of the right.

The Independent foreign correspondent Robert Fisk is a particular cause celebre. His articles on the Middle East are gleefully pulled apart, to the point where "fisking" has come to mean the sport of intellectually trashing any piece of left-wing "propaganda". John Malkovich was asked whom he would like to fight to the death. Robert Fisk and George Galloway, he replied.

Even luminaries such as Paul Krugman (who is leftist by US standards) can't escape blog wrath. Sullivan and his acolytes glory in highlighting the smallest inconsistencies in Krugman's popular New York Times column. And this is the blogger's way: like raptors, they hunt in packs, gain momentum, pick enemies, vent spleen, and never, ever, hold back.

These blogs do not have large direct readerships: InstaPundit clocks only 40,000 readers a day. But many readers run their own blogs; others are political or media professionals. So a growing community is aware of whatever most irritated Sullivan today. This in turn creates what the legal theorist Cass Sunstein calls "cybercascades", reaching millions of readers with ideas, in this case associated almost exclusively with the right. They are democratic dynamite: private networks of information, unchecked by sensible debate. The aftermath of 11 September increased the cascades. Blogging became warblogging; the community became indignant cheerleaders for any madcap Bush anti-terrorism scheme. Attempts to question were given a vigorous fisking.

It is no exaggeration to say that the current disconnection between America and Europe over Iraq has been exacerbated by the blasts of bloggers. As Sullivan posted recently: "The problem with the Europeans is that they didn't experience 9/11 themselves, have a history of appeasing terror, and so find the new doctrine . . . an implicit indictment of their entire foreign policy record of the last decade." Such bunk goes uncontested. It is almost part of the mainstream. But what explains it?

Importantly, they got there first. Sullivan had an established following for his journalism, and took his chance when the first blogs appeared. The medium lends itself to short, sharp, witty commentary of the sort often associated with raging libertarians. But although polemicists of the left - Robert Reich, James Carville, Al Franken, John Pilger and others - may have websites, they have stayed out of the blog fight.

The right would argue that it had nowhere else to go. It sees itself as the victim of a vast, left-wing media conspiracy. Because it is barred from the mainstream print media (not true, but play along), it seeks "underground" new media.

Should the left worry? Definitely. The blogsphere is an example of Willard Quine's coherence theory of truth: that things are true if they agree - or appear to agree - with other things that are held to be true. Right-wing bloggers are thus creating their own world, in which their truth exists often without debate. And the same may be about to happen in the UK. The journalist Stephen Pollard, the only British political blogger on the left, notes: "There are plenty of new British political blogs. And they are all - all - on the right."

But political blogging is in its infancy here. It remains up for grabs. Got a computer? Got a view? Get blogging. There is a war to be won.

James Crabtree runs two blogs: www.voxpolitics.com and www.theisociety.net