Matthew Norman: Clegg is helping his enemies to annihilate his own party

So Nick Clegg, you think you're Achilles? Well, I know Achilles, I've read about Achilles in The Iliad, I've seen Brad Pitt play Achilles in the movie. And, let me tell you, Mr Clegg, you are no Achilles. The above is, of course, witlessly adapted from Lloyd Bentsen's crushing put-down to Dan Quayle, and while Mr Clegg can spell "potatoes" in all his 47 languages, he is every inch the laughing stock that hapless vice-president ever was, and then some.

What can he have been thinking? What possessed him to hide from the jeers while David Cameron stood at the Dispatch Box on Monday? What hubristic madness blinded him, so that he confused absenting himself from a debate of historic national import with the duties of a party leader and Deputy PM?

The thing about Achilles was that, as the mightiest warrior of his time, he could get away with sulking in his tent. The other Greeks thought him a spoilt crybaby for withdrawing himself and his Myrmidon troops from battle after Agamemnon, that cunningly opportunistic leader of a fractious coalition, deprived him of his heart's desire. But they soon caved in, returning his confiscated Trojan lover and begging him to return, because they needed him desperately.

Who the hell needs Nick Clegg? Well, clearly David Cameron thinks he does, calculating that the toddler temper tantrums are a price worth paying to sustain a Tory Government it is now embarrassing to refer to as a Coalition. Any replacement, be it Chris Huhne, Tim Farron, Simon Hughes or preferably Vince Cable, would feel obliged to follow a more conventional timetable, and fight valiantly for his principles before, and not after, Mr Cameron incinerates them. And Ed Miliband needs him even more. Apart from the protection offered him by the comparison with Mr Clegg, Labour is virtually guaranteed two thirds of previous Lib Dem voters if he remains in his post.

When all his electoral value is to his rivals, you'd imagine he would have the residual sense to know it's time to make Monday's temporary retirement permanent. He is now a lethal liability to his party's chances of requiring more than a small dinghy to transport the Lib Dem parliamentary presence across the Styx to the political underworld after the next election.

This blunder comes not in isolation but as part of a continuum. Despite his 15 minutes as a hero during the last campaign, his poor judgement was apparent long before he sat beside his master nodding frantic approval of the tuition fees he had so fervently opposed. In February 2008, he led his Myrmidons out of the Commons in protest at one of them being suspended after becoming aerated when denied a debate in which the Lib Dems planned to make a tactical appeal for a referendum on EU membership. Somewhere in there may lie a tiny irony. Another may have arrived a few days later, when the man who had promised that the Lib Dems would show their cojones over Europe castrated himself by ordering his MPs to abstain from voting on the Lisbon Treaty.

You might have expected those twin fiasci and the ridicule they inspired to have taught him that public sulking over European disappointment is seldom a masterstroke, though people often say that things come in threes. The essential thing for his party, if he still cannot see the hopelessness of his position, is that it doesn't allow him time for a fourth.

Curiously for such a well-balanced and likeable chap, Mr Clegg's career in power has mirrored none so clearly as Gordon Brown's. Gordon started so well that for a few weeks it was almost possible to kid yourself that realising a dream could empower a politician to brush off his character flaws like flakes of dandruff. But no one really changes in middle age, and Mr Clegg's flaws resurfaced as quickly as Gordon's had a few years earlier.

Where the latter's strengths were tactical mastery of political process and the drive to impose his agenda by (deranged) force of personality, Mr Clegg's decisive weaknesses are innate cowardice in defending his principles and perplexing tactical ineptitude.

The one area in which Mr Clegg was supposed to be fantastically astute, after a previous career as an MEP and EU trade negotiator, was in the fine nuances and Byzantine complexities of Brussels negotiations. Yet he so completely failed to read the runes, and anticipate how reciprocal Anglo-French intransigence might play out, that he gave Mr Cameron his imprimatur to use that mythical "veto", assuming that he would never actually be goaded by Sarkozy into pressing the trigger.

Awoken before dawn by the news that the gun had been fired, and that he had therefore committed the crassest tactical blunder in British political history since an imaginary Tory candidate stood in Golders Green on the Arab League ticket, at first he reprised his nodding dog act from the tuition fees disgrace. How he compounded his error by failing to foresee his colleagues' fury, God alone knows. Finally, it penetrated his sleepy head that he was complicit, through naivety, in a monumental betrayal of his party's most cherished belief, and that once again, Mr Cameron has made as big a fool of him as he has made of himself.

The extent to which this decent, intelligent man is out of his depth becomes inescapable. It is like watching a Victorian gentleman nobly respecting the Marquess of Queensberry's rules against a bare-knuckle fairground fighter – for all the surface poise and elegance, Mr Cameron is a ferocious scrapper on the blind side – who is gouging and rabbit punching him to death. I would add "hitting him below the belt", but with this peevish court eunuch what would be the point?

Just as with Gordon, there comes a moment in the life of a zombie leader when even those of us who adore politics for the gloriously vicious combat sport it is can no longer bear to watch. It came two days ago when Mr Clegg eschewed taking it on the chin to spend more time with his infantile sense of grievance.

If Mr Clegg sees himself as Achilles, brooding with romantically fearsome rage at being dissed by his commander-in-chief, he strikes me as another Homeric character altogether. He is his mortal enemies' quickest, surest conduit to the final annihilation of the Liberal Democrats, and their most most devastating gift. He is that Trojan Horse, and the day cannot come soon enough when he morphs into Ajax.