Under the Cosh, this may as well have been called, which is perhaps a little unfair, but you didn't have to speed-read it inside a week. It's not that this is a bad book. It is, in many ways, a good book: King's take on the America of Bush and 9/11, a nation on the verge of environmental and moral collapse. But it is, in so many other ways, too much, too big, too long. And too Stephen King.

Even diehard fans of his peerless imagination, of whom there are justifiably many millions, will struggle with the sheer heft of the thing: it's like carrying around something which is simply wrongly weighted for a book, a hefty dead cormorant or some such, and after a little while it begins to feel like carrying around a grudge.

King writes short stories splendidly well and has won awards for them. He has a bizarre little idea and everyone goes: "Oh of course, why could I never think of that?" Normally, however, he judges it just so: the power of the idea is equalled by the length of the execution. Here he's got the proportions wrong.

It is a fine idea. A small, typical Maine community finds itself, one day in the very recent past, cut off from the rest of the world by an invisible, impenetrable dome, or Dome. Yes, I know The Simpsons Movie did the same thing, but King reportedly began this 25 years ago. It is not, this being King, a gentle awakening. The dome simply appears one second (miles high, as we soon learn, and extending way below bedrock), and when it appears some hands are pulling out root vegetables and are thus severed, while little planes crash and leave sinister smudges.

Some of the early goosebumps come when the eventual hero, Iraq vet Dale "Barbie" Barbara, and a new chum from the other side of the barrier (they can hear each other, and a little air can pass between them, but that's it) walk for miles in parallel, trying to find if it's a wall with an end or a well, a dome.

From then on, we're along well-established lines, from Nevil Shute and before, inflected with the contemporary terror of environmental crisis: the air going bad, the water running out. The community goes to pot. A very bad fat man takes over, with guns. Religious zealots go (even more) mad. The mob, almost, rules, thwarted by a few good oddballs. There is comradeship, love, repentance.

There is also paranoia, blame and violations in the name of "security", and it's not hard to see the satire on Bush's America, especially when the main route on the "safe" side of Chester's Mill is the 119. These last few sentences seem terribly reductive; it's better than that. King reads widely, writes widely: there are glancing references to everything from Eliot to Melville to his fellow thriller writer Lee Childs. The existential explanation for the dome is beautifully managed, warmed up and hinted and, yes, keeps the pages turning.

The horror is also there. Partly, simply, through the language: King loves language and the way people use it. Take the terrible nastiness of redneck "Junior" Rennie, exposed when a girl's robe falls open and we hear his thoughts about her "breeding-farm", her "goddamn itchy breeding-farm that was all the fuckin trouble". His father, the real baddie, Big Jim, meanwhile, never swears but uses biblical euphemisms instead.

So much is sinister, so much plotted with grand intentions and lucid resolution. But despite the book's cover boast that it "took over 25 years to write", it turns out, in King's own honest words  for this is an honest and a brilliant and busy and moral man  at the very end, that it was written between 22 November 2007 and 14 March this year. He had a grand idea, a long time ago, then hammered it out recently in a year and a half. He could have done it as skilfully in a month and saved us the hernias.

Stephen King's new novel is predicated on, and takes its epigram from, the song "Small Town", one of country-singer James McMurtry's savagely compressed and contemptuous indictments of American life. "It's a small town, son," McMurtry sings, "and we all support the team," inflecting the words with a bland, overbearing oppressiveness. Not content with this quiet pressure cooker, and determined to write what he describes as "a book that would keep the pedal consistently to the metal", King drops a dome over his small town  Chester's Mill, not far from the infamous Castle Rock  and clamps it there so we can watch what happens.

The dome is invisible: a "force field", perhaps. Its appearance, as witnessed by drifter and frycook Dale Barbara, is sudden and unforgiving. Barbara, recently beaten up in the car park of the diner where he worked, and anxious as a result to leave town, is forced to change his plans. A human being can touch the field without harm, he discovers, though there's a faint accompanying tingle: but electronic devices, including iPods and pacemakers, explode the moment they come near it. Unyielding and impermeable, especially to energetic solid objects, it conveniently permits sound, light and radio waves to travel through. It has a slight permeability to moving air; fire hoses trained upon it, while they do not wholly penetrate, produce a faint refreshing aerosol within.

Soon, everything in Chester's Mill is going to hell. Two dozen children suffer seizures. The air inside begins to heat up. The surface of the dome collects dust and pollutants, diffusing the light that falls on it so that sunset spreads "across the western sky like a great poisoned egg". Outside, the US administration gathers its wits; but an attempt to gain access by bunker-buster comes to nothing. And if the air beneath the dome is heating up fast, the political atmosphere is heating up faster still, as, increasingly panicked, the trapped townspeople, their food and propane running out, begin a grisly search for that central concept in Stephen King's fiction, the community's aggregate or "secret" will. Alternate centres of power grow up around Dale Barbara  who turns out to be more than the drifter he seemed  and Big Jim Rennie, the town's Boss Hogg.

Big Jim drives a Hummer like a "rolling coffin". On his desk he keeps an autographed photograph of Tiger Woods, his "tinny testimonial to smalltown prestige and smalltown power". To Big Jim, the dome is just another political opportunity. He upsizes the police force and provokes a food riot to increase his control over the population. Meanwhile, Dale Barbara and Julia Shumway, editor of the local paper, organise a demure resistance. The ideological oppositions clarify. Soon everyone, from the sickest child to the kindliest golden retriever, will have a part in the mystery play, as Under the Dome, like The Stand before it, works through its vast biblical collision: liberal morality and a moderate green sensibility versus greed, corruption and fundamentalism.

Under the Dome is nearly 900 pages long, and has a cast to match. Characters maintain separation by clearly announcing their basic traits whenever they appear. Dale Barbara regrets a crime he committed during the second Iraq war, while Mrs McClatchey smiles wanly and carries a picture of her husband. Julia Shumway, though "Republican to the core", drives a Toyota Prius. Piper Libby, the apostate minister over at the Congregational church, must control her temper, because "If she didn't, it would control her". It's hard to say whether these Post-it notes are addressed by the characters to themselves, or by King to himself, or by both of them to the reader.

But if Rennie is the epitome of this method of characterisation, he's also King's cold-eyed assessment of the Bush regime, just as Chester's Mill is his guess at what America might look like over the next generation, as resource failure, pollution and global warming take their toll on a closed system run for profit. "Who in their right mind," muses one character on the third day of the crisis, "would ever have expected this sudden contraction of all resources? You planned for more than enough. It was the American way. Not nearly enough was an insult to the mind and the spirit."

Under the Dome builds slowly but in the end delivers all the grue and brisance you'd expect of an apocalypse in a bottle. By page 45, someone has been scalped by a broken windscreen  "a huge flap of skin hung down over her left cheek like a misplaced jowl"  and the pedal is indeed to the metal. People's jaws are broken with stones. Entrails stream out of abdominal cavities. Brains resembling breakfast cereal spatter over floors and ceilings. Various forms of transport crash into the dome and explode. A man explodes. There are many different kinds of guns, and by the end everything but a nuclear weapon has been set off, in a kind of localised Stalingrad of the hick mind. There are aliens, home-made radiation suits and a necrophiliac with a brain tumour. There's murder by golden baseball and, for good measure, a methamphetamine lab big enough to supply the whole of North America, a "General Motors of meth". Sometimes it works, and sometimes you feel King's heart isn't entirely in it.

To keep the reader focused, King regularly quotes from "Small Town", but he can't match the ironies that undermine the complacency of McMurtry's audience. Where McMurtry's songs encourage everything in life to bleed into everything else, the us-and-them oppositions of Under the Dome are too well differentiated, too overtly polemical. In a three-minute song you can deal in fractured glimpses; in an 800-page blockbuster you must render unto plot all that is plot's. Someone has to be the bad guy. Someone has to pay. Causes are all present and identifiable, and evil is rendered safe by overstatement.

M John Harrison's latest novel is Nova Swing (Gollancz).