Perhaps it should come as no surprise given the vast range and number of posts he has held, but who would have guessed that Ken Clarke's final government position would be the novel one of Cabinet Fig Leaf? Ah, well, at first sight at least, though less so on second, Kenneth has the requisite girth. As fig leaves go, few if any political titans of our lifetimes have been as beautifully designed by nature and perfectly shaped by over-indulgence to cover the blushes of a Prime Minister who exposed himself yesterday as even more cringefully weak than one had previously thought.

Now unmistakably the petrified hostage of the Tory right wing which had long and hysterically demanded Ken's removal as Justice Secretary, David Cameron may not be impotent yet. But the deafening message of this reshuffle is that his power over the more distempered of his backbenchers and their tabloid compadres has drained alarmingly. He now lacks the strength to keep in place one of very few senior ministers not merely deemed to know what he's doing, but of whom the punters are by and large fond. After William Hague, whose popularity is inflated because the Foreign Office raises him high above the fray, Ken and Vince Cable have the highest approval ratings in the Cabinet. If you imagined that these statistics would offer the Tory right a handy tutorial on the paramount need to colonise the centre ground, where on earth have you been living these past 15 years?

The PM knows all this much better than I do. For all his failings, he is a very clever man, and as such comprehends that any chance of winning most seats at the next election, let alone a majority, relies on seducing more swing voters than he did last time. He also understands that none of his colleagues is capable of dog whistling (or more aptly cat-napping) an aura of moderation, common sense and humanity by his presence alone than the portly jazz fan from Nottingham. Which is, of course, why Mr Cameron has retained Ken Clarke without a portfolio. The old boy had no wish for an easy life and very much wanted to stay and complete his reforms, but had to settle for what is clearly less a real job than a comfy berth in the political departure lounge.

It won't work. The public is also far from stupid, and knows a cynically manufactured sinecure – the equivalent of sidelining a distinguished academic ancient with the meaningless title of Emeritus Professor – when it sees one. Much will be spun about the cruciality of Ken's new role as George Osborne's wise Nestor on economic policy, and the incalculable benefits of having his experience on various Cabinet committees. To this the most considered response is an (Albert) Steptovian "Yeuuurggh, cobblers".

In one context, and one alone, can the neutering of Ken Clarke be regarded as sensible. While it is a monstrous error guaranteed to hasten the retoxification of the Conservative brand, it makes sense solely as a defensive manoeuvre to ward off a serious challenge to Mr Cameron's leadership. In that case, it qualifies as a smart short term tactical move... and in the luminescent absence of any long-term strategic thinking, it would be childishly Utopian to expect more from him than that.

Yet it doesn't half make him look a pitiable wretch, and there it is in excellent company. Reshuffles conceived to project prime-ministerial strength have a habit of being interpreted as signs of fatal weakness, and not since the Night of the Long Knives has this theory of unintended consequences had a triumph like Cameron's Morning of the Plastic Spoons. Trying to replace Ken with Iain Duncan-Smith was one thing. If Mr Cameron wanted his predecessor but one ranting and raving against human rights legislation, while ceaselessly parroting Michael Howard's foolish mantra that "prison works", that too makes sense in the limited arena of buying a few flattering headlines from the Daily Mail and Sun. Failing to persuade IDS to accept the job, on the other hand, and having hurriedly to give it to the first alternative right winger in view, Chris Grayling, shines a halogen lamp on the diminishment of the PM's authority as much as his managerial incompetence.

As for Jeremy Hunt's promotion to Health Secretary, all you can really do with that bombshell is borrow from the late Sidney Vicious, and indeed the even later Eddie Cochran, and mutter an awed, "Wow. That's somethin' else." The brazenness beggars belief. Here, we find Mr Cameron taking the murky, back-alley culture of the mafia pay-off, and putting it centre stage by overtly rewarding a mirthless public joke for acting as his human shield over Rupert Murdoch. Armando Iannucci would dismiss such a sub-plot as too satirically broad to be worth the attentions of his stiletto nib.

The last PM to spring a shock on such a scale, by recalling Peter Mandelson from Brussels in what proved a brilliant if desperate survivalist ploy, was Gordon Brown. While Mr Cameron was trying to finalise this reshuffle, it must have chilled him to learn that the Paralympics crowd had rousingly cheered Gordon, whose reputation has recovered amazingly, as he presented some medals. The same crowd ferociously booed Mr Osborne, who survives despite the degradation of his own stock. The British are not natural givers of the bird to even the more despised of our national figures.

Being no fool, the PM will have discerned from these contrasting receptions that, on austerity and other fronts, the centre of gravity is shifting leftward. He has journeyed in the opposite direction, meanwhile, with a range of Cabinet appointments (chief whip and party chairman among them) dictated by the pressure to placate a rebellious right which still cleaves to the curious belief that the Nasty Party kept being crushed at the polls purely because it wasn't quite nasty enough.

When a Prime Minister lurches one way just as the centre ground moves in the other, a chasm is created into which that PM – bolster his immediate position though he might – must inevitably fall in the end. And when that PM is so feckless that he demotes one of his most talented and popular colleagues, for the week's breathing space before Mr Hunt's puppetmaster, Mr Murdoch, finds something else to bully him about, a precipitous fall into the void is exactly what he deserves. David Cameron's reshuffle has revealed how limp and shrunken he has become, and even the mighty Ken Clarke isn't a large enough fig leaf to hide his shame.