It's not just the Tories gathered in Birmingham who know Boris Johnson is unstoppable now It can never be easy being the Prime Minister with the Mayor of London on the rampage, but seldom harder than yesterday when the latter addressed The Boris Johnson Fan Club (formerly, the Conservative Party Conference) in Birmingham. No one threw their panties at the blond bombshell, admittedly, but the cutaway shots eloquently told the amorous tale. There, amid the beatific smiles of renaissance painting disciples gazing up at the Messiah, sat David Cameron, reddening for once with embarrassment rather than rage as the Adoration of the Mop Top Magus gripped the hall. If the Cameron response was one of grossly disproportionate hilarity, no wonder. When Boris praised him for taking the tough decisions, "not least coming along to hear this speech today," he spoke the truth. It must have been torment for the PM, cast as the terminal patient overdosing on laughing gas when the undertaker popped along to measure him up for the coffin. With the zoom lens examining his features for the faintest revelatory twitch, what could he do but guffaw hyenically at Wildean references to his failure to translate Magna Carta? Mr Cameron is a very clever man, but in this game of No. 10 hide-and-seek, a dunce would know that Boris is coming to get him, ready or not. Mind-blowingly surreal as this strikes me, having spent years chronicling the omnibuffoonery, the question now is less whether Boris will become Conservative leader than when. Wise old birds like Norman Tebbit and Kenneth Clarke mutter that he is utterly ill-suited to the post, while former colleagues in this industry emit mirthless cackles of derision at the prospect. Yet the Tories desperately seek a deus ex machina to sweep down from Olympus and make everything all right, and those enraptured faces swept away any residual doubt that logical objections to the post-Olympics Boris can be filed away in the trash can of irrelevance. The speech was good rather than great, with Boris reining in the stirring rhetoric for fear of seeming too rapacious, à la Gordon Brown's "best when we are Labour" job application of long ago. So rather than search for the G-spot, he contented himself with stroking a few erogenous zones (enterprise, strivers, hop-on buses, enterprise, and, oh, did I remember to mention London 2012?). But he did it cutely enough to make it plain why - given a choice between a Bullingdon Boy they think (wrongly) despises them and another they think (even more wrongly) loves them back - the faithful would plump deliriously for Boris. Combat politics is a visceral business, and whatever the flaws in his private and public lives, he resonates on a gut level like no one in memory. And so, taking account of the hard facts (approval ratings and his proven appeal to otherwise Toryphobic London voters) and the more nebulous Borismania, he must make a political calculation - one that strikes me as less tricky than it may strike him. The conventional rules seem to state that he must sit it out until his mayoral term expires in 2016, after the next general election, what with him having solemnly sworn not to return to parliament until then and the old Heseltinian cliché about crowns and assassins. But what, I hear you sardonically shriek, do conventional rules have to do with Boris Johnson? Not a bleeding sausage. The Boris who promised Conrad Black he would not become an MP while editing The Spectator, and then did, will hardly feel constrained by a silly little thing like his word of honour (never an obsession with him). As for the Heseltine template, you can dump that in the bin with all the other objections rendered meaningless by his escape from the politico-gravitational pull into the meta-political ether. Boris's only dilemma can be whether to strike before the election or after it, and this will primarily depend on whether Mr Cameron and the economy are visibly recovering this time next year. If, as seems favourite, both continue to struggle, and a Tory defeat looks inevitable, he would be daft not to descend via a by-election and launch the putsch. As a proud classicist, he will know the original Greek for the Delphic Oracle's injunction, "Know thyself." He must have the self-awareness to see that he is anything but cut out to be leader of the opposition - a rigorous, tedious job, with all the draining drudgery of laying tactical traps for the Government and formulating coherent policy. He'd self-destruct inside five months, let alone five years. As PM, with little more to do than delegate (always a strong suit), crack a few gags at PMQs, and project unfounded optimism with the shambolic charm that seems irresistible even to those who should know better, he would fancy himself to shine. Under the shadow of an imminent Labour victory, he would have every right to challenge Mr Cameron, and if MPs fretting about holding their seats voted him into the final two, as they would, the membership would give him a landslide. Factoring in the wretched history of long-term Tory leadership frontrunners, this may be one of those occasions on which the superficially safer option - waiting patiently for defeat in 2015 and receiving a coronation - is in fact incomparably the riskier choice. His craving for power is not in doubt, and whether he has the courage to knife his old schoolmate in the front we may find out soon enough. If it sounds crazy, this burgeoning suspicion that Boris Johnson will be Prime Minister - well, in truth, crazy doesn't come close. Every now and again a national figure emerges who somehow seems less a human being than a force of nature. Thatcher was one, Blair another, and Rupert Murdoch another. Boris is his generation's Act of God. It can hardly be a benign deity that would saddle us with Boris Johnson, and he will blow himself out in the end. Until then, the PM has no response, proportionate or otherwise, to a tornado that could pick him up from his bed and spin him all the way to Munchkinland, while keeping the house for itself.