Something so weird has happened to Alan Johnson lately that, despite the evidence of his own mouth, you have to suspect him of being reliant on a mind-altering drug.
The old Mod has never touched anything illegal, whether Class A, B or C, so he claimed a few years ago during one of those sporadic bouts of conflated hysteria about politicians and dope. As a young guy he did the sex and rock 'n' roll, he said, but never the middle element of the holy trinity.
Now here he is, less than six months into his tenure as Home Secretary, and his personality has palpably changed in the way many of us will have observed in an addicted friend or relative. The sweet-natured, laid-back darling we remember is replaced by a capricious bully of the sort who instantly jettisons someone from their life for telling them a truth they'd rather not hear.
Much has been written about his brutal dismissal of Professor David Nutt for having the impertinence to inform the public about the science of which he is a world-renowned expert. The argument about the comparative dangers of cannabis, ecstasy, alcohol and tobacco is so old that I won't give it what the Newspaper Columnists (Arrestable Punning and Imbecile Word Play) Act, 2003, precludes me from calling a rehash.
The debate will rumble on for a few decades, you'd optimistically imagine, until the self-defeating idiocy of "the war against drugs" – the lunacy of wasting untold annual billions on policing and prison places by driving those who need medical help to crime – becomes apparent to all.
Until then we must endure the perpetuation of this insanity by governments in unceasing thrall to right-wing newspapers and columnists whose malevolence is surpassed only by their scientific ignorance. This is Britain, shining beacon of political duncery to the world, and no amount of railing about the lunacy will change the weather in a climate zone where a man who has spent 30 years poring over peer-reviewed research papers is silenced for sharing his knowledge.
The only surprise in this latest eruption is to find Mr Johnson donning the Jacqui Smith Memorial Dunce Cap. Frankly – and this cannot be said of any cabinet colleague – we expected better of him. We expected, at the very least, that he'd find exhibitions of the brittle, reflex authoritarianism that cracked the Nutt beneath his dignity. But then dignity is invariably a casualty of dangerous addiction, and Mr Johnson has become dependent on something really nasty. He is hooked on ambition for power – something else he claims never to have inhaled – and no drug is more destructive to the personality than that.
With hindsight, the clues were there long ago, when alone among trade union leaders he supported scrapping Clause 4 a few months before Mr Tony Blair repaid him with a safe seat in Hull; when this worshipper of John Lennon's lyrics voted strongly to invade Iraq; when as an education minister he single-handedly rescued the top-up fees that would close off the university escape route to so many born into poverty and urban deprivation like himself.
What changed Mr Johnson's life, he once said, wasn't education (famously he left school at 15 without an O-level) but the Beatles. When adolescent Al was supporting his first family by stacking Tesco shelves, John, Paul, George and Ringo were his idols and inspiration. At the same time, they were campaigning for legalisation of the cannabis without which so much of that life-changing music would never have been created.
Now no one travels the long and winding road from school drop-out, via postman and militant trade unionist, to uber-Blairite fixer without a measure of ambition. But my guess is that he thought he could handle it. It's the biggest cliché in the Class A lexicon, is it not? They always think they can enjoy the odd line or crystal or ministerial promotion without falling prey to dependency. Eventually, of course, the craving takes over until they will steal anything (the sentiments of certain leader columns) and sell anything (their political souls) for a fix.
Exactly when Mr Johnson realised he needed the ultimate hit that is the leadership of his party and possibly (timeframe permitting) his country, is impossible to pinpoint. But judging by two other recent interventions, it happened during the last few months.
As recently as July, he actively courted the fury of the right-wing media by proclaiming himself a supporter of mass immigration. "I do not lie awake at night worrying about a population of 70 million," he elegantly rebuked the Home Affairs Select Committee. "I'm happy to live in a multicultural society where we ... welcome those coming to live and work in this country." In July, he was just as happy for the United States to welcome Gary McKinnon, the computer hacker with Apserger's, into a state penitentiary for the crime of embarrassing the Pentagon.
Now he has suffered one of those infamous druggie mood swings on these issues, both especially dear to the campaigning heart of sections of the press. Quite suddenly – rightly, I believe, but for absolutely the wrong reason – he has "stopped the clock" on Mr McKinnon's extradition. And he has issued that unilateral mea culpa on New Labour's behalf over the policy of barely checked immigration. Given that the evidence hasn't changed a jot about either issue since July, the confluence of these U-turns must be read as unmistakable evidence that he is ever more urgently positioning himself to succeed Gordon Brown.
That Alan Johnson would be a better leader of party and country than Gordon is hardly the point (the same goes for Katie Price, Pete Doherty and Kai Rooney), and electorally he may well remain Labour's second best hope, after Mandy, of restricting the Tories to a slim majority. By no means is his abrupt conversion to such retrograde right-wing verities his banana moment, because it doesn't make him look geekily and sensationally ridiculous.
What it does make him look is just as venal, opportunistic, vacuous and unlovable as the rest of them. That seductive image of the chilled rocker who talked about ultimate power as if it were a fungal infection, whose music biz son insisted his old man would much rather be in the Super Furry Animals than Number 10, has been shattered. Where before he suggested an engagingly louche minor associate of Arthur Daley, he now looks more like a cousin of the Krays.
Disloyalty is the lifeblood of politics, and no fan of this captivating combat sport would have it any other way. But self-betrayal is much darker, and the vista of someone who spoken eloquently of growing up on a rough London estate beset by alcohol-fuelled violence and domestic abuse pretending to regard cannabis as the greater social ill to ingratiate himself with newspapers is particularly dismal even in a government so suffused with professional turncoats.
"I haven't got the ambition and I haven't got the self-confidence and I haven't got that real aching desire to lead." Mr Johnson told Desert Island Discs listeners in 2007. Who knows, maybe back then he really did think he could use Red Box without getting hooked. If so he knows even less about drugs than we thought, because on the addictivity scale, as Professor Nutt himself learned only last Friday, that one gives heroin and crack more than a run for their money.