A change is taking place in the economic weather; and change in the political weather must follow. To suit an altering climate over Britain the Official Opposition needs a change of focus, of message and of tone. The key Tory messages of cuddliness and generosity should now leave the spotlight.

The new Conservative language should be about waste, maladministration, extravagance, incompetence and drift. The new idea should be the need in hard times for rigour, severity and unsentimentality. Sheer necessity should be part of the backdrop to every Tory speech about the economy and public services.

The whole ethos surrounding a party leadership with ambitions to topple the present Government must be of a ruthlessly businesslike instinct to cut the fat, strip waste, sack the incompetent and pare down public administration to its essentials. David Cameron, George Osborne and their team should present themselves as a hit squad of top-flight company doctors, sent in to rescue a flabby and flailing corporation on the verge of insolvency. Tories should not fear or duck the implication that there will be victims, sacrifices and cuts. Shadow ministers should not shrink from the impression that some people are going to hate this new Tory government's guts.

The public are ready for this. Guessing the shape of tomorrow, a shrewd Opposition today would breathe its impatience to make a bonfire of new Labour vanities. Unsqueamishness would become the new cool. The cartoonist Martin Rowson's image of David Cameron as a flower-picking, chubby, lace-cuffed, purple-pantalooned butterfly-chaser is potentially fatal, and exactly the picture he needs to disavow. A shard of austerity must enter the Tory leadership's aura.

If I were in charge of shaping the new Conservative message I should within six months be wanting to hear from focus groups that if the Tory party were a sauce it would be Tabasco; if it were a garment it would be a hair shirt; if it were a physic it would be smelling salts; and if it were weather it would be a bracing northerly breeze.

Mr Cameron and his team have shown enough heart already for a sense of essential decency to have sunk in among those disposed to give him a hearing. Now for head. Now for unsentimentality. This is not the same as unfairness. Younger Tories may forget that a burning public resentment at the perceived unfairness of Labour Britain was what drove millions of non-unionised, non-council-subsidised, non-inflation-proofed British voters of all classes into the voting booths in support of Margaret Thatcher in 1979. State generosity was unfair.

Let me offer instances. The Shadow Cabinet should arm itself with a shortish list of big and unfair wastes of taxpayers' money, and big failures of public administration, and hammer their contents home until we can almost recite them - just as in 1978-79 people recited Tory mantra about the Winter of Discontent, the unburied dead, the Spanish practices of public sector unions and the subsidy of state housing.

What might be their equivalents today? Gordon Brown's tax-credit system; the abuse of incapacity benefit; the New Deal programme; the near-doubling (to such modest effect) of spending on the NHS; the proliferation in central and local government of advisers, consultants, inclusivity officers, media teams, communications experts and tin-headed, jargon-spouting Directors of Strategic Narrative; the legion of students pursuing pointless courses in woolly disciplines; the complete failure of the national drug-rehabilitation programme; the dissipation of the Department for International Development's too-swiftly expanded budget into a thousand pointless club-class officials' flights to and from Third World capitals. The staggering failures of the Department for Environment, Food and Rural Affairs.

To these could be added a new critique that the Conservative Party is ideally placed to develop: the ruinous waste of public money on private sector contracts. As the party that (it could claim) understands business, the Tories should be offering a devastating account of the way naive Brownite ministers who don't understand the City have been hoodwinked into parting with vast sums of money for dubious deals offered by canny operators who saw these gulls coming.

Mr Cameron should talk about Metronet to his backbencher Richard Bacon on the Commons Public Accounts Committee. The whole PFI story is riddled with scandal and the Tories could make a virtue of being the party who thought up the idea, understand it and now see it - and the taxpayer - abused because Labour ministers don't.

Tory spokesmen may protest that they do raise these criticisms; and it's true that most have had a mention, though the party's failure to confront the scandal of 21st-century PFIs head on is fairly disgraceful. But time and again the opposition case is muffled - as when a speaker places his hand over his mouth - by the Tories' nervousness about suggesting that they would ever take anything away from anyone. An unspoken disclaimer heads every opposition pronouncement on government waste: “Although we're the cuddly party and wouldn't dream of hurting anyone...” The disclaimer blunts whatever attack follows. Prefacing the attack with “Because we're the cruel-to-be-kind party, and there can be no gain without pain...” would sharpen it. Nor should the taxpayer be exempt. If money is tight there can be no handing taxes back either.

Those who think that in the current climate the electorate are likely to admire open-handedness in politicians are simply wrong; time will prove it. A better case against my argument can be made by those who maintain that mere frugality can never be the driving force of a programme for economic survival and regeneration. If in opposition the Tories cannot develop different and more cost-effective structures for delivering good public services, then snipping costs around the margins will not rescue them.

That is probably true. But I'm not sure how interested (or trusting) the voters are in opposition ideas for structural reform. Shadow Cabinets must develop such policies but they may not get much attention in the shop window. Neither Margaret Thatcher nor Tony Blair won power with a very clearly defined manifesto. What was better defined was the instinct and personality of the aspirant leadership.

Mr Cameron has done enough - and he had to - to make his party appear more amiable. He has gone a long way to banish a spiteful Tory image, standing him in good stead if he now explains that his party has some harder things to say. He must prepare to disappoint the liberal press and a handful of new-found friends in saying them. A vocal minority will yell that the mask is off and the old Tory Adam is out.

The rest, who do not doubt his liberalism, will lay aside with relief their doubts about his hard-headedness. Lean and mean are the watchwords, and it is time to harden up.

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