Why is the left so nasty? Why are politicians brought up in a Labour movement that bangs on about brotherhood and sisterhood so consumed with dislike and suspicion of one another? If we believe the newspapers, most members of the cabinet and many other Labour MPs are almost perpetually at daggers drawn, or at least simmering with mutual resentment. The rejoicing at Peter Mandelson's fall, and then the whooping at Charlie Whelan's resignation, are only the latest examples. And on this subject, mirabile dictu, the newspapers are certainly to be believed.

It is a tradition. One of Labour's mid- century heroes was Ernie Bevin, the trade union leader turned foreign secretary. Bevin has many earthy, pearl-encrusted lumps of wisdom for today's party. But new Labour has taken only one Bevinism to its heart: his retort to Attlee's remark about Herbert Morrison (a fellow member of the 1945-51 government) being his own worst enemy. Bevin snarled: "Not while I'm alive, he ain't."

So it goes on; a cosy party of worst enemies ever since, like a convoluted Sicilian family whose feuds are repeated generation after generation. John Prescott, the nearest we have to an Ernie Bevin, waged a long and malicious semi-public campaign against Mandelson, Morrison's grandson, constantly mispronouncing his name as "Mendelson", and once comparing him to a crab. Class hatred? Bevin would have approved. Meanwhile, I don't suppose there's a single important minister who someone else hasn't told me is lazy, corrupt or paranoid. Then, just as we were all digesting the Mandelson affair, we got an eerie echo of today's events, courtesy of the 30-year delay on cabinet papers being released. Alongside the modern grudge matches, the press simultaneously relayed the latest news from 1968 when Ray Gunter resigned from the Wilson government because he couldn't stand the "the bloody middle classes and intellectuals within the cabinet" and, in particular, "that bitch" Barbara Castle. There's a ringing feeling in my ears.

It was also the year of the fall of George Brown and he, too, was a famous hater. There's a story about him marching up to Len Williams, an ex-general secretary of the Labour Party, and one of Brown's enemies, who had just agreed to become the governor-general of Mauritius. Did the post involve wearing a plumed hat, Brown pleasantly asked. Yes, said Williams modestly, yes it did. "Well," said Brown, "I hope your fucking feathers fall out."

Is Labour fated to relive, government after government, the poisonous personal feuding that undermines the party's moral authority? And if so, why? Before the election, Tony Blair was warned that his biggest headache in No 10 was likely to be relations between his senior ministers - Gordon Brown and Mandelson, Brown and Robin Cook, Cook and Mandelson, Prescott and Brown, and so on. He laughed the idea off.

He probably isn't laughing now. This might be less damaging if it weren't for the striking similarities with cabinet feuding in the now-reviled Wilson years. The publication of Dick Crossman's cabinet diaries in the mid-1970s was a watershed because, for the first time, it tipped the cabinet's mutual animosities and bitterness into the public arena. For people outside the closed world of Whitehall and the press lobby, it was a moment of revelation. I can vividly remember my history teacher's shock and amazement as he read out entries to us.

Harold Wilson, realising what damage was being done, tried to get the rules on publication of cabinet diaries toughened up - to little effect, since the Castle diaries and then the voluminous Benn diaries followed. Crossman himself had gone to the root of the matter years earlier, when he wrote in his back-bench diaries in 1956: "When the Tories are in trouble they bunch together and cogger up. When we get into trouble, we start blaming each other and rushing to the press to tell them all the terrible things that somebody else has done."

Tory coggering had rather fallen out of fashion by the time of the Major government - or rather, when they did cogger up, someone was usually left dead on the Westminster marble afterwards. Under post-Crossman glasnost, we know a lot about how much the Tories came to loathe one another, as their European battle raged on. The Lawson and Howe resignations, then Margaret Thatcher's own bitterness at her removal ("treachery . . . treachery with a smile on its face") had set the scene for John Major and the "bastards". And, at the height of the battle, the hatreds were deep and real. I recall walking with Norman Lamont down Whitehall after his resignation; Tristan Garel-Jones, spotting us, immediately crossed four lanes of moving traffic to seize Lamont by the arm and warn him in bloodthirstily unrepeatable terms what he would do to him if he went public against Major. There are scores of similar stories still knocking around. These are vendettas that will probably last for a generation. Purely from a journalistic point of view, I do hope so.

But this should be of little comfort to the present cabinet. For one thing, the Tory feuds were mostly about policy, very important policy, and were perhaps unavoidable given the difficult choices facing the Thatcher and Major governments over Britain's destiny. Second, those governments were destroyed by them. Third, in the political mythology, Tories are supposed to be nasty - but efficient. They don't go on about fraternity and comradeship, do they? So again, why are Labour people so nasty to one another? Is it that lefties are simply disagreeable? That they are either cold, intellectual renegades from the upper middle classes, driven by dislike of their own backgrounds; or working-class politicians who despise the middle-class people they have to mix with?

Could be: academia, trade unionism and left-wing activism are all poor training-grounds for niceness and mutual respect. Academia is famously poisonous and rivalry-ridden. The clash between factions in the unions produced a culture of leak and counter-leak that was brought into government again in 1997. And the climate of oppositionism around the left in the late 1970s and early 1980s was particularly grim.

There is a hilarious and uncomfortable account of those times, Things Can Only Get Better: eighteen miserable years in the life of a Labour supporter (Doubleday, 1998) by the comedy writer John O'Farrell, which rings bells with many of us who were there.

"The media coined phrases like 'Hard Left' or 'Militant Left' but 'Very, Very Boring Left' would have been more accurate," writes O'Farrell. "It wasn't the left-wing-ness in itself that was the problem, it was the excessively bad-tempered and humourless way in which the left argued its corner. . . We had somehow got it into our heads that a period in opposition meant that we were now opposed to everything."

Some of the things O'Farrell decided were right wing were "flowers, fish knives, ladies' hats, power steering, Wellington boots, the county of Surrey, Donald Duck, conservatories, and any girl's name that ended in the letter 'a' . . . One of the people that I lived with in Exeter decided that smiling was right wing. He pretended to be miserable as a sort of political statement throughout the early 1980s."

I remember people like that. And, let's be honest, there have been humourless, life- hating lefties since the Levellers began hawking Socialist Peasant around Middlesex; and certainly since Robespierre gave purity a bad name. It is the psychological profile described by George Orwell: "The typical socialist is a prim little man with a white-collar job, usually a secret teetotaller and often with vegetarian leanings."

There's nothing wrong with a bit of puritanism, particularly in January, but it is connected to a left moral fundamentalism that can easily turn nasty. O'Farrell again: "We all believed, or rather we knew, that socialism would solve everything and that any particular individual who happened to be to the right of us was responsible for all the world's problems and therefore we hated them. As far as I can remember, just about everyone I knew on the left was like this. Angry, negative and totally un-self-aware. And Tony Benn gave the British left its chance to vent its ugly spleen in public. At regular intervals the news would feature Benn supporters booing Denis Healey, pelting him with missiles and picketing his meetings. They seemed to hate him more than they hated Margaret Thatcher."

It wouldn't be hard to see how a party whose left-wing members had marinated themselves in self-righteous hatred (remember Nye Bevan's 1948 proclamation of his "deep and burning hatred of the Tories . . . they are lower than vermin") might carry this right to the top. But there are a couple of problems with the thesis that Labour, despite the generosity of its vision, is somehow nasty by nature. First, most of the people feuding in cabinet had nothing to do with that era of ultra-leftism. Brown, Cook, Prescott - they hated it at the time as much as O'Farrell hates it in retrospect. Second, if it were true, then the Labour right would be conspicuously nicer than the Labour left, and that also isn't so. Denis Healey had a brilliant line in invective. Think of his famous remark about David Owen: that when he was born, the good fairy had given him everything - good looks, brilliant intelligence, luck. "Unfortunately, the bad fairy made him a shit." Nor would it be entirely true to say that, for instance, dear Gerald Kaufman was a kindly old soul. Or that Mandelson himself pursued a "speak no evil" policy.

No, I think the real answer is a mixture of all this, and something else, which is simply that Labour has spent too long in opposition. Out of power, the big egos kept themselves warm by dreaming of top jobs. But these are, by definition, very limited indeed. The same was true of the early Wilson years and of the Attlee government. What has changed is that the constant competition among desperate souls, for a handful of slippery perches, now takes place in front of a media that is itself more desperately competitive, and that exaggerates the smallest differences. This produces a bitchy climate that few ministers can resist. Since Tony Blair, who doesn't seem to have a malicious bone in his body, is one of the few who can resist, perhaps he can use recent events to draw a line under the warfare. Enough people have gone. Enough others have been frightened. Perhaps this really is grow-up time.

Everyone I've been talking to tells me so. They know that, in the long term, this behaviour is deadly. On the other hand, I can't help thinking that at the back of their minds there is a piece of advice that rings louder still. It's Stalin: "There is really nothing more delightful than carefully plotting a trap into which your enemy in the party is bound to fall, and then going to bed." Ho, ho, comrade, ho, ho.

Andrew Marr writes for the "Express" and the "Observer"