An anniversary approaches whose resonance is solemn. Thirty years ago Britain was in the last week of the general election campaign that first sent Margaret Thatcher to Downing Street. Her victory looked likely but not assured.

Although I had been working for her for two years in her Westminster office, I was fighting my own little battle for the Derbyshire constituency whose Conservative association had selected me as its prospective parliamentary candidate. As a carpet-bagging new kid in the constituency (and criticised for it) this was, for me, a nervy time, although we were defending a stonking majority there. “How are we doing?” I asked our bluff association chairman.

“Could be pretty tight,” he replied.

My heart missed a beat. “But we've all been running such an energetic campaign,” I said. “Am I coming across so badly?”

He grinned. “Raise your eyes from the doorstep! I'm not talking about your election. I'm talking about ours. In the country.”

I reddened. My party's national campaign had gone largely above my head. The householders I was doorstepping had had time to follow the news more closely than their candidate. I certainly hadn't committed to memory our tome of a Tory manifesto, or the deftly worded treatise, The Right Approach to the Economy, by my Research Department boss Chris Patten. But from what I did absorb, I don't remember there being anything much about privatisation, doubling VAT or the cruel salami-slicing of welfare benefits that was to follow. Even on the broad theme of “taming the unions” the extent of our declared ambitions was unclear. I cannot call to mind a single Conservative plan during that 1979 campaign that Labour succeeded in turning into the sort of scare story that a worried voter could hit you with on the doorstep, demanding: “Oi! What about this, then?”

Yet when her Government was formed, the going got tough and Mrs Thatcher got going, there was hardly a protest that could not be knocked down with the retort: “You asked for it. Or the country did.” Critics conceded as much in the wording of a range of bumper stickers widely sported in the 1980s and 1990s: variations on the theme of “Don't blame me - I didn't vote Tory”. They blamed their own countrymen. An idea of inestimable value to the Tories in difficult times had been born - that the policies we were following had been a national choice.

And so, in the months before that election, Mrs Thatcher's Tories pulled off a remarkable combination. They had and ate their cake. Studiedly vague (even to themselves) about what they were going to do, they had subliminally implanted in the national mind so strong a sense of the kind of thing that they were going to do that when they did it, the charge that the electorate had been hoodwinked never really stuck.

There was one reason above all others for this achievement. In her own person, Margaret Thatcher just breathed what she was about. Her manifesto inhabited her. Her instincts were transparent. So much so that she has since been widely credited with a more wholesale paring back of public spending than she ever achieved.

This is why you will hear the story - the last ousting of a Labour government by a Tory opposition - cited both by those who urge David Cameron to be upfront about the pain, and those who warn him to avoid Gordon Brown's trap. Both sides are right. Mrs Thatcher was indeed cautious about the detail of future policy. What she was upfront about was her instincts - not her plans.

And today? After the Budget's confirmation of soaring national debt, Mr Cameron finds himself caught between the case for spelling out how he'll handle it (thus securing an honest mandate for government) and the case for not giving Mr Brown opportunities for target practice before the election.

I believe Mr Cameron too can square the circle; but that it will have to be a more conscious stratagem than Mrs Thatcher needed. With her - almost a comic-strip character in the public mind - personality could almost substitute for policy.

With Mr Cameron it cannot, because he has spent the past four years in deliberate soft focus. Anxious to show his party as compassionate as well as hard-headed, his public image has been subtle, perhaps ambiguous. Now some harsher associations will have to be sought and hammered home.

George Osborne, the Shadow Chancellor, has made a well-aimed start with the word “austerity”: a shrewd expression because its connotations are virtuous although its implications are unkind. And he and Mr Cameron can go some way simply by harping on the themes of national sacrifice, tough choices and painful times to come. But on its own this is all a bit negative, and will continue provoking the response: “Which sacrifices? What choices? Pain where?” Silence will invite the suspicion that they know but won't tell us.

If (as is probably closer to the truth) the Shadow Cabinet doesn't really know yet how a Conservative government is going to move back towards balanced budgets, if the scale and nature of public spending cuts and tax increases has yet to be decided, then there's much to be said for making a virtue of the search.

The keynote - their key mood - should be inquisitive. The subtext should read: “Everyone can now see that to avoid national bankruptcy, radical changes will be needed. It must go beyond trimming, cheeseparing and ‘efficiency' savings. Unlike Labour, we have come to terms philosophically with this. So, we believe, have the voters. We have a year, and, unlike Labour, we have agreed the question. Now for the answers. We have some suggestions already but we need more. We're in the market for ideas.”

The era of smart-casual young think-tankers in cool-framed specs with schemes for spending public money is over. The overarching question now is: “How can we cut?” The era is upon us of sharp-minded, compassionate but unsentimental people, not necessarily political or party people, who can propose and develop such thinking; and the hunt for such people and ideas is on.

To the question “What are you going to cut?”, Mr Cameron's implicit answer should be: “You tell me. No, really: you tell me. We Conservatives have the steel to do this, and you know we must. Now let's talk openly, without scaremongering or posturing, about how and what and when.”

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