The headline in Tuesday's Times was arresting; “‘Help yourselves,' Cameron tells the fat and the poor.” Momentarily I thought the Conservative leader was inviting the needy and overweight to a second plate of pudding. David Cameron must have come far with his Tory detoxification strategy, for “‘Help yourselves,' says Thatcher” would never have invited that reading. By self-help she meant one thing.

And no bad thing, either. So why did I feel a touch of nervousness about Mr Cameron's speech in Glasgow on Monday? Because slowly over my life, and resisting all the way, I've learnt that for the CentreRight, moralising when you take things away from people poorer than you is desperately tricky territory.

I understand the temptation. Anyone who has canvassed for the Tories in bad times and bad places, will understand. I must have spent months of my life canvassing: as a parliamentary candidate in Derbyshire, and in other constituencies; but most memorably in Wandsworth in London in the 1970s, standing for the borough council. Tories have always been proudly insistent that there are no no-go areas for vote-seeking. The party has a sincere belief that every corner of Britain deserves to hear what it has to say. It knows that though only a minority of the poor are receptive to the Conservative message, that minority is substantial and stalwart, and should not be written off. So I have knocked on thousands of doors in scores of council estates in some of the worst parts of London and the Midlands.

And all who have done this will agree: For a Tory, a day of canvassing sympathisers in “bad” areas leaves you urgently convinced that an uncompromising right-wing message on the undeserving poor is a vote winner.

The deserving poor do not like the undeserving poor. They feel very deeply that when you don't have much, and you struggle to be honest, to look after your family, to keep up standards and to stay in work however badly paid, the handouts available to the lazy, the greedy and the dishonest are a kick in the teeth to everything you've stood for. Householders will show you their neatly kept patch of garden or privately purchased top-of-the-range front door, and point to the mess made by their indigent neighbours; and shake your hand with a grim “we're voting Tory come what may”, or just “God Bless Maggie”.

On doorstep after doorstep you hear this: bitter and passionate. You feel you've made a discovery. You return from your foray into the Nelson Mandela Estate fired with the idea that our party must take a stand for these people; that the “real” Britain is more reactionary about the welfare state than the liberal newspapers think; that it is a Tory duty to represent (as the socialists never will) these estimable but forgotten citizens, trying to be good in places where it's much harder to be good than in Eaton Square. You feel the strongest urge - backed now by direct evidence - to speak out for the virtuous poor.

Resist it. You are not the man, and your party's voice is not the voice, to do this.

Anyone would think, from the sudden interest on both Left and Right in the writings of the modern American thinker Gertrude Himmelfarb, that the argument about the deserving versus the undeserving poor has only recently been discovered. But it was a dominating sociological debate in the 18th century, and right through the 19th century too.

Anyone would think it was socialism that sold the pass on the moral society and began (as Himmelfarb has it) “de-moralising” society by alleviating distress regardless of how caused. But it was really the Victorians (preachy as they now sound) who developed the idea of undiscriminating provision. Re-read Dickens, Trollope, the parable of the Prodigal Son. Look at the social outreach work inspired by the Wesleys and the nonconformist churches. The inherent conflicts between rewarding virtue and trying to alleviate all distress are ancient, profound and logically insoluble.

Logically insoluble, but not incapable of rough-and-ready resolution in keeping with the spirit of the age. Nobody today thinks the alcoholic should be left to die unattended in the gutter just because he chose to drink. But nobody today thinks that in the NHS queue for transplant organs he should have the same priority as the young mother with a genetic liver disorder. A balance must be struck. It will be struck according to moral sensibilities that to a very great extent we all share, but should be careful about declaring too noisily.

Nobody needs to be more careful than the Tories. Say everything we can and should about the Cameron Conservatives' social inclusiveness and compassion, but when we've said it, it remains the case that the Tories are seen as representing the achievers, the would-be achievers, and the already achieved in society. There isn't any way someone who has made it can moralise about witholding help from someone who hasn't without striking a displeasing note.

I've tried. Very hard. As an MP seized with the conviction that if people would do more to help themselves they really could manage, I lived on the dole for a week in Newcastle more than 20 years ago, in a bid to prove it for Granada's World in Action. I heated only one room, bought in bulk, shopped around for seconds, kept warm with gardening, chose an inexpensive sport, etc - and I could have succeeded. But when I sensed the anger I was arousing among nice Geordies I met, not so much because they disagreed with what I said but because of who I was to say it, it dawned on me that I, a sleek young Tory earning ten times what they did, should not be saying it. Not without giving offence. It was a matter of taste, really. So I decided to fail in my bid. In doing so I rescued my political reputation and made my media career.

So I'm afraid this is actually a rather cynical column: not about policy but presentation; not about what you do but about the reasons you give.

If you're a Tory restricting health or social benefits, don't give a reason. Or not a moral one. Just say the money's run out. Your own moral sense may urge you to offer a moral justification. Resist it. Passing a beggar, don't stop and tell him you don't think money is what he needs, however decent the impulse to explain. Just walk on with a friendly “Sorry but I can't”. He'll know why. Everyone knows why. Trust the unspoken moral instincts of the people, which remain strong and fairly merciless about the undeserving, but don't articulate. You, Mr, Mrs, Ms, Lord or Lady Tory, are not the person.

The next Tory chairman will be fat - count on it: I've guessed: seriously fat. All power to him: he's a talent. All power, Mr Cameron and George Osborne, to your drive to make the nation, and the Tory manifesto, leaner and meaner, for so you should. But no more speeches about overreating.

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