Right-wing Tories apparently still believe this is a make- or-break issue in a potential leader. Even Mr Clarke himself - a man whose generous waistline and trademark cigar testify to more old-fashioned vices - is expected to put himself beyond suspicion.
At a meeting with the Thatcherite 92 Group, Clarke was asked by some upstart MP: "Have you ever used any Class-A drugs?" The Big Beast raised himself up and gave a dignified snarl: "If it is of any use to you, I have not taken cocaine - to be quite clear." But he went on to warn the 30 MPs before him that they were on a slippery slope: "If you start asking personal questions, it does not stop."
Thus, with one swipe of his mighty paw, Clarke saved his young rival from any further embarrassing questions, established himself as the protective paterfamilias of the Tory tribe, and bolstered his reputation as an easy-going, bonhomous man of the world.
Cameron must be kicking himself. Being rescued has made him look younger and less experienced than ever - and it hasn't even cleared his name. By the time Clarke came along, it was too late: Cameron had already issued a partial mea culpa on television. "I did lots of things before I came into politics which I shouldn't have done. We all did," he told the BBC's Andrew Marr. Cautioning against a witch-hunt he added "I didn't spend my early years thinking I mustn't do certain things because I might become a politician. I didn't know I was going to be a politician."
It's a fair point. The standards of personal piety required of modern politicians are unreasonably high - and quite out of sync with the cultural norm. No one of my generation (which is also Cameron's) could have got through university without encountering drugs, and very few without trying them.
I am, by nature, extremely cautious and law-abiding. The first time I saw someone snorting cocaine, at a student house party, I started weeping and noisily imploring her to desist - "It's disgusting! It'll ruin your life!"- until my friends became so embarrassed that they bundled me into a bedroom and locked the door, promising to let me out only when I had calmed down.
But almost anything can start to seem normal if it is ubiquitous enough. In the late 1980s, when Cameron and I were coming of age, rave culture revolutionised British attitudes to drugs. Ecstacy flooded into the mainstream, bringing all sorts of other Class As in its wake. By the time I left university, I had tried most of them - though never with the hedonistic élan of my peers. (I refused to go to raves - far too noisy and crowded - and always secreted emergency phone numbers all over my person, just in case.)
In anyone under 40, then, a history of drug-taking is no more revealing of character than a history of cigarette-smoking. It is just something that most people do when they are young, and stop doing (if they still can) when their constitution starts to conk out.
The political orthodoxy - and the reason right wingers continue to ask these awkward questions - is that anyone morally lax enough to take drugs in their youth is bound to be dangerously lenient in government. But that is no longer true - if it ever was. The most powerful former coke-snorting pot-head in the world - at least according to the lurid accounts of the late Hunter S Thompson - is George Bush; a man not known for his soggy liberalism.
Why, then, doesn't Cameron just make a clean breast of it? The answer, I suppose, is that he is too avid a politician. His style mentors - Bill Clinton and Tony Blair - are both masterful equivocators, and both profited by fudging this issue. In the heady days of Cool Britannia, Blair giggled knowingly with coked-up pop stars, but never confessed to dabbling himself. Clinton, of course, claimed not to have inhaled. Through such delicate hypocrisies, both men managed to align themselves with yoof while - just about - maintaining their gravitas as statesman.
Honesty in such matters is altogether more perilous. For one thing, it can easily be mistaken for boastfulness. Even Boris Johnson, the most likeable politician of his generation, became temporarily embarrassing when he confessed on a television chat show to having smoked pot. When Mo Mowlam came clean, she was dismissed as a tiresome show-off. Years ago, I switched on some zany late-night television show to find Tony Banks MP demonstrating how to roll a joint. All the hairs on my head went rigid with vicarious stage fright. Like all such attempts to suck up to the young, it bombed.
As a Tory, Cameron has a particularly fine line to tread. A cultural chasm exists between his own generation and that of the party membership. Until he has the leadership in the bag, he cannot afford to ruffle the whiskers of Colonel Outrage of Tunbridge Wells. A little sophistry is needed to bridge the gap.
Perhaps, after all, both Clarke and Cameron have got it right. In politics, there are some questions that are best left unanswered - and better still, unasked.