"Knock, knock." "Who's there?" "Mo." "Mo who?" "Politics is a cruel business."

Remember Mo Mowlam? She used to be the most popular politician in the country. Admittedly, that could be damning with faint praise, but she was indeed the people's Mo. When she announced that she was leaving the House of Commons, there may not have been weeping in the streets, but there was still a pretty universal sense of a great talent gone to waste. She is still, after all, relatively young (she was born in 1949) and might have expected a good few more years near the very top.

Instead, she is now remembered, if at all, for having plunged, in what seemed like a matter of weeks, from being Labour's greatest asset to being its most embarrassing problem, and the victim of a series of pretty foul briefings. Indeed, the reaction to her memoirs has been overwhelmingly hostile. Until I read Momentum, I assumed much of that was due to the combination of her taking the Daily Mail's shilling - an unforgivable sin among most of the left-liberal chattering classes - and . . . well, and what? The Mo Mowlam story, and the hostility that her name now provokes, remains an enigma. Many of her former political friends now openly despise her, and I've always found it difficult to pin down quite why. None of the accusations that were most commonly levelled against her when she was in office - that she had little grasp of necessary detail, that she believed her own popularity myth - is worse than the personality defects of almost any politician. None of them explains the venomous comments of some of her colleagues.

Misogyny certainly provides a partial explanation. Next time you find yourself in the company of a genuine new Labour man (he should be under 25), wonder aloud why it is that there are so few women in top positions. I guarantee that the response will be a variation on the theme of "there aren't enough good women around". Try it and see.

In September 2000, after being given yet another briefing against Mowlam by one of the "kids" (the sharp-suited researchers, advisers and lobbyists who have chosen to make their careers in the Labour Party - their generation's equivalent of the 1980s Tory Boys), I wrote a piece arguing that a misogynist streak was integral to the psyche of many new Labour types. Spend any time in their company and you soon start to hear how Ms So-and-so is "thick", how Mrs Whatshername has "made her career horizontally", and how Ms Thingummy is "a total bitch". The afternoon after my piece appeared, I was at a seminar held by a Westminster think-tank. I was approached conspiratorially not once, not twice, but three times by kids, each of whom said more or less the same thing to me: it's not misogyny, not at all. They really are thick. Another telephoned me to tell me why I was wrong. "Look at Harriet Harman. She's fine on TV, but give her a real job to do and she just can't cope. Over-promoted."

Take Mo Mowlam herself. Have you ever heard her described as Dr Mowlam? But the abuse that was directed against her was far, far worse than ignoring her academic qualifications. In February 2000, the Independent on Sunday led with a story about how its political correspondent had been told that Mowlam's brain tumour had left her "without the intellectual rigour" to do her job: "The illness appears to have affected her. She doesn't seem to be able to do the job in the same way."

The sheer poison of the invective against her was, and remains, breathtaking. It was unrelenting. But reading Momentum, it becomes a lot clearer why she became so hated by her colleagues. Mo Mowlam is a monumental egomaniac. You would, it's true, be hard put to find a half-decent politician who didn't think himself or herself the answer to the country's problems. But the scale of Mowlam's conceit would embarrass most others.

She writes of how badly she was treated by Tony Blair. Having been forced out of Northern Ireland to make way for Peter Mandelson, she was dumped into a non-job at the Cabinet Office. Without a hint of irony, she tells us that she had been to see the Prime Minister to discuss her next position: "He wanted me to take a big spending department, such as Education or Health . . . I felt that going to a big spending department would be like going out of the frying pan into the fire . . . I would like a go at [Defence Secretary] and that would put me in a good place to try later on in my political career for Foreign Secretary . . . He wanted to give me Health . . . He started arguing it was my duty to him as the PM, to the party and to the country to do what he wanted to enable Labour to get a second term. The trouble was that these arguments no longer had any effect."

Er, hold on. Two of the biggest jobs in government weren't to her fancy, so she issued her own demands to the Prime Minister. She couldn't give a damn about working for Labour's second term, so long as she had a fun job. If this is what she is prepared to write for public consumption, what on earth was she like in private? She is so lacking in self-awareness that she doubtless has no idea that the only person to emerge from her account with credit is the supposed villain of the piece, Tony Blair, who not only indulged her, but did everything he could to keep her in government.

This is a sad book. I have always been on Mowlam's side, nauseated by the tone of many of the comments made about her. But even though the specific content of such comments may have been unjustified, the picture which emerges from her own hand is of a politician so wrapped up in self-importance that she deserves not sympathy, but scorn.

Stephen Pollard is a political commentator and broadcaster