On April 1, I came across no less than four media April Fool jokes that involved Peter Mandelson and his dome. Why does this junior minister and his edifice attract such ribaldry and occupy such a prominent, usually demonic role in, if not the nation's psyche, then certainly that of the media?

The best of the gags was on Radio 4's usually dreary Midweek programme. A commercial chemist claimed to have created a potion which stimulated secretions of the "happiness brain chemical" serotonin, the one that is increased by taking Prozac. The new invention could be sprayed onto crowds creating instant collective well being.

Apparently Mandelson had attended an experimental episode of Noel's House Party in which half the crowd had been sprayed with the wonder drug but not the other half. He was so impressed with the resultant good cheer in the sprayed section that he immediately signed the boffin up to perfume the minds of Dome visitors in his "Mists of Time" (or some such fabrication) zone, through which we would supposedly all first pass on our way in.

This joke contained two themes common to most Mandelson-Dome stories: sinister coercion and doubts about the "futile" Dome's ability to entertain. The other spoofs, in The Guardian, Mirror and this paper, also contained these themes. What lies behind these projections onto Mandelson and his Dome? Do they reflect deeper currents in the country at large or are they merely eddies of bitchiness on the surface of London's shallow media and political worlds?

Mandelson goes to great lengths to attract publicity and makes no secret of the fact that he is an ambitious politician. His politics also make him a target. As he told me in a 1995 television interview (broadcast last year, the one in which he became tearful), both extremes of the spectrum want to discredit him: "I get it from both sides. I get it from the Tories and I get it from the hard left of my own party."

He has certainly made his fair share of enemies and by all accounts can be a ruthless and cunning adversary. He can be hectoring and arrogant in his treatment of media who have angered him, although only his most hostile enemies deny that in person, socially, he is usually playful, amusing and charming.

But most of these points could be made about Blair's press secretary Alastair Campbell. None of them explains the scale of the opprobrium against Mandelson or the power of his symbolism as a Bad Thing - devious, creator of the vacuous dome, a joke (dubbed Mandy by many journalists).

But homophobia could. The fact that Mandelson has not made a fuss about his homosexuality - why should he? Blair does not bang on about being heterosexual - makes some suspicious that he has other "secrets" that he is "hiding".

Hardly a day goes by without The Guardian, self-styled as The Thinking Gay's Paper, referring to "Mandy". Playful and amusing though this may be, the fact is that Mandy is a woman's name and Mandelson is a man. Likewise, numerous articles in all papers refer to the fact that Mandelson has not yet decided on a gender for the statue in the Dome's Body Zone. The implication that he has not made up his mind about his own sexuality is unmistakeable.

But this is not the core cause of Mandophobia. The main reasons are his relation to Tony Blair and the ambiguities of a Labour politics apparently disconnected from its socialist roots.

Opinion polls estimate Blair's personal popularity to have been sky-high for a long time now, but if we feel passionately about anything, there is always ambivalence. If Tony is All Good, then we must be repressing the Bad we also feel, because nothing is all good. What do we do with the split-off, unconscious feeling? Peter Mandelson is that Bad, a role made explicit in Blair's famous comment that he would know he had succeeded when the "Labour Party has learned to love Peter Mandelson".

This explains a good deal but there is also a much more serious political point about Mandophobia. Unlike Blair, whose real political beliefs are obscure in the extreme (it is not even clear if he is an Anglican or closet Catholic), Mandelson unambiguously stands for something: the centre right of Labour politics eschewed by his grandfather, cabinet minister Herbert Morrison. Here at least is a target to shoot at because Blair's "New" Labour leaves nearly everyone unsure where to aim.

Tories cannot quite believe that Labour seriously has taken over Mrs Thatcher's twin project of robbing the poor to give to the rich and of deregulating to make it easier for crooks to fleece the vulnerable. They cannot find much evidence that Blair poses a threat so far, but an uneasy, ill-defined feeling in their (privatised) water tells them that something does not quite stack up. Lord Rothermere a Labour peer? Rum old business.

Left-leaning people are equally mystified. The hard left, like Ken Livingstone, make no bones about their discomfort but the less extreme would like to believe that Tony and Gordon's hearts are in the right place. The trouble is, they too see little sign that he will claw back some of the family silver handed over to the utilities, that Murdoch will be reigned in, and so on.

Since Blair's words give no indication whatsoever as to what is actually in store for us (and probably will not do so until after the next election), this leaves a gigantic void, one that is itself symbolised by a space which has received a ludicrously disproportionate amount of media attention: the gap underneath the canopy of the Dome.

Behind the issue of what Mandelson is going to put there lies the question of what New Labour is actually going to do when it really gets down to business (after the next election). The hostility to the Dome, is really repressed hostility towards Tony Blair.

At present, Peter is Tony's whipping boy. But if the Labour party and the media did learn to love Peter, what would they do with their hate for Tony?

'Britain on the Couch - Why we're unhappier compared with 1950 despite being richer' by Oliver James (Century, pounds 16.99).