Matthew Norman: Now that the Prime Minister's lost his charm, what's left?

I write in a state of high anticipation a few hours before a scheduled trip, with an officially adolescent family member, to see The Avengers. In this warmly reviewed film, an alliance of Marvel superheroes is formed to save the world from a malevolent Norse god. Iron Man leads an all-star team featuring Thor, Captain America, Hawkeye and the Black Widow, but the figure I expect to find most intriguing is the Hulk. Any character who instantly metamorphoses from a placid, genial man into a raging maniac must have special resonance for the student of Westminster life.

David Cameron's ferocious anger is itself in the process of mutating into a central issue of his premiership. Asked a week ago about Jeremy Hunt and the economy, he turned crimson rather than green, and looked more fit to burst the blood vessels on his cheeks than the buttons on his shirt. So it was on Monday. Ostentatiously livid to be summoned by the Speaker, he lost his rag again and did something cosmically stupid. He attacked Dennis Skinner in a manner which might have been computer designed to illuminate his greatest weakness.

Reports of the senescence to which Mr Cameron crassly referred seemed exaggerated when, for the second time in five days, Mr Skinner effortlessly pierced his protective shell. Following last Wednesday's "When posh boys are in trouble, they sack the servants", the superhero known to Bolsover's comic fans as The Beast unleashed another fearful zinger. "Whenever the Culture Secretary is in the firing line," he said, "it prevents the bullets from hitting the Prime Minister." The PM's response was not only a shameful affront to the dignity of his office, but revealed that Mr Skinner's bullet had found its mark. "Well, the honourable gentleman has the right at any time to take his pension, and I advise him to do so," he snapped, and no one lashes out as spitefully as that unless seriously wounded.

We mustn't get too prissy on behalf of a Labour MP who has dished it out, in that prolier-than-thou way, for decades. I wouldn't dream of patronising him by hinting that, pushing 80, he is too doddery for the demands of combat politics. That suggestion we may leave to Mr Cameron, and if it delighted his back benches, that speaks only of the idiocy of Tory MPs in confusing the enfeebled failure to keep his temper with a show of strength. It was a horrendously ill-judged line, and the clearest sign yet that he is in mortal terror for his survival as PM.

It also cutely vindicated the previous Skinner jibe. This was Cameron in gouty, Edwardian country toff mode, splenetically dismissing a superannuated beater who cheeked him on the grouse moor. In a happier time and place, he'd have sorted the matter, you felt, with a private word to his estate manager. "Look, Snodgrass, that uppity old man with the tweed jacket and red tie, you know, the one who does those dreadful music hall monologues at the staff summer picnic? Skinner, you say? Well, get rid of him. I don't mind decrepit retainers hanging around – always pleased for cook to give them a cup of tea and a bun, what, what? – but I don't want to see this one again. See to it that he takes his pension and buggers orf, there's a good chap." Here was Cameron reduced by his wrath to embracing the very thing he is most desperate to escape: that lethal perception of born-to-rule patrician aloofness.

Dennis Skinner doesn't deserve the contempt of cocky whippersnappers who never did a proper day's work. He deserves the respect due to both his seniority and his background as one of nine children of a miner sacked after the General Strike of 1926, and a miner himself. The symbolism of this Prime Minister savaging a man who dragged himself out of monstrous poverty, by what Clause 4 knew as hand and brain, speaks for itself.

This sort of nuclear peevishness – a deafening echo of Rupert Murdoch's reaction to being riled by Robert Jay QC; honestly, the two seem supernaturally conjoined – is potentially catastrophic to a leader whose appeal relies almost solely on affable charm and the equanimity he feels entitles him to condescend to others with "Calm down, dear". When it comes to questions of character, politicians are damaged not by their acknowledged flaws, but by failings which directly contradict the persona they trade on. Bill Clinton survived Lewinsky because everyone knew him for a randy dog, and George W Bush was re-elected despite being a halfwit because half wittedness was a major part of his shtick. Had the Rhodes Scholar been exposed as a moron and the born-again moralist as an adulterer, each would have found his brand fatally contaminated.

As an erstwhile public relations operative, it cannot be that Cameron fails to appreciate the paramountcy of protecting his brand. He simply can't restrain that poisonous temper. "He's a guy struggling with two sides of himself, the dark and the light," observed actor Mark Ruffalo of the Hulk, whom he plays in the movie. "Everything he does in his life is filtered through issues of control." Lack of self-control may be essential to Dr Bruce Banner's alter ego in seeking to save the planet, but it is hardly an appealing or reassuring trait in a politician flailing about to save himself.

Although you cannot like Cameron when he's angry, it is difficult to picture him as the Hulk or any of Marvel's creations. The contrast with the real Captain America is especially unflattering. On Saturday night, at the White House Press Correspondents' dinner, Barack Obama, who has spent four years calmly enduring attacks on his integrity to make Mr Skinner's look like a kiss on the nose, grinned with every sign of genuine pleasure as comedian Jimmy Kimmel roasted him to a crisp. Our Captain Insensible would have had a seizure.

His decline has been astoundingly swift. In January, he answered another Skinner question, with an attempt at humour, albeit in the wonted ageist style, by citing a glance at him as a handy alternative to taking his kids to the Natural History Museum to see the dinosaurs. Three months on, the rage leaves him no retaliatory weapon but vulgar abuse. With each week that goes by, the odds against him recovering lengthen alarmingly, and if he is ever to be cast as a superhero now, we'll have to wait for Marvel Comics to introduce Iron Pyrites Man. Just a few weeks ago, he glistened in the distance like 24-carat gold. As we move ever closer to the real David Cameron, the more worthlessly ersatz a commodity he comes to appear.