John Rentoul: Cameron has struck some clever poses. But is there any substance behind them?

It sometimes it takes a day or two for the significance of turning points to sink in. No, I do not mean the loans-for-peerages imbroglio, which, however distasteful, will probably have little effect on the date of Tony Blair's departure. I mean David Cameron's response to the Budget on Wednesday.

With his leadership of the Conservative party barely 100 days old, during which time he has hardly put a foot wrong, Cameron made his first big mistake. One of his best lines as leader, on his first day in the Commons, was to turn aside from asking a question of the Prime Minister to chide Hilary Armstrong, the Labour Chief Whip. He accused her of "shouting like a child".

That is precisely what he did this week. His delivery was rushed and too loud, trying to compete with the noise of the Labour benches opposite rather than letting the microphones do the work. That is, of course, just an error of technique. Practice will fix it. But it was the childishness of the content that was the real issue.

The end of the end of Punch and Judy politics has exposed him as a student debating politician, unserious and insincere. With the George Osborne, his shadow Chancellor, looking too pleased by many of the jokes (he probably wrote them), they looked like two young men playing at politics. It was completely the wrong approach to take to the occasion and the man.

The Cameron strategy is clever and basically right, in that it is the only one that offers the Tories any hope at the next election. He must concentrate his fire on Gordon Brown and portray him as the obstacle to reform, while sticking as closely as possible to Blair in order to widen Labour divisions. But personal abuse never works well, in politics as in life.

Simply calling Brown the roadblock to reform doesn't persuade people that it is so; it is an argument that needs to be developed. Cameron must know that, whatever people think of Brown as a potential Prime Minister, he commands substantial respect as Chancellor. And worse, from Cameron's point of view, is the fact that the anti-Blair bias of the liberal media tends to mean that Brown gets a relatively easy ride. That is not to say that the Cameron strategy of bear-hugging Blair and doing down Brown is the wrong one, merely that it needs to be implemented with subtlety and maturity rather than vulgar abuse.

The way to approach Brown, therefore, is in a spirit of bipartisan reasonableness and then to test - by asking politely open-minded questions - whether his instincts are statist, controlling and bureaucratic.

It is not as if there are no chinks in the armour of the ironclad Chancellor, after all. They could be widened, but they need a sharp point, not a hammer. It is remarkable that he should be borrowing so much, at a time of - as he proclaims - sustained economic growth. Equally, Brown's aspiration to raise spending per pupil in state schools to the same level as in private schools was as irresponsible an uncosted pledge as any of those for which the Liberal Democrats have been attacked, by Brown and Blair, over the years.

But to attack Brown on these fronts would require the Conservatives to propose lower spending or higher taxes. The Chancellor's record on green issues is defensive and stealth-based. The idea that £45 extra road tax on school-run monster trucks will have any more effect in deterring petrol use than a Greenpeace cardboard wheel clamp is nonsensical.

Yet Cameron's green rhetoric, the centrepiece of his dazzling debut as leader, has not been followed through. Indeed, after only three months, much of Cameron's positioning has already been exposed as contradictory where it is not actually empty.

In an article for this newspaper last year, Cameron implied that he was against new roads and runways. A week later he said that "Britain now needs a concerted programme of road building" - albeit coupled with a system of road charging. In the same article, he said he wanted to "end the bias towards wind power" in order to allay concerns about "wind farms in upland areas" - in other words, all windmills to be built in sheltered lowlands.

No wonder, then, that Brown was able to win the argument on the day simply by taking refuge in the climate-change levy - a tax on energy-intensive industrial users of which most consumers know nothing except that it has a name that spells out its purpose. Cameron declared on day 27 of his leadership:

"We should not just stand up for big business, but stand up to big business when it's in the interests of Britain and the world."

So far, however, his opposition to the climate-change levy might have been drafted by the CBI. He complains feebly that the levy is not directly related to carbon content of the fuel used. If he applied the same principle that he did to the Education Bill, he would support the levy on the basis that it is at least a step in the right direction.

Cameron's advisers said last week, on the occasion of the completion of his first 100 days, that his leadership was now entering a "new phase". The Conservative party would discuss the "aims and values" statement that Cameron published in February, and this debate, and the vote on the document later this year, would prove to people that the change in the party is real.

As if. In the absence of spontaneous social movements, change in parties has to be driven from the top, and so the question for Cameron's next phase is whether he can give some substance to the clever poses he has struck so far.

Peter Mandelson offered some not entirely disinterested advice last week. As communications director before the 1987 election, he said, "I tried to effect these changes by means of a spray job: putting out the red rose instead of the red flag, changing the colours, the communications, the style of the party with words rather than deeds. And the country saw through it."

There is no question that Mr Cameron has been a success so far. From an average opinion poll rating of 32 per cent that has hardly varied for 14 years, the Conservatives have averaged 37 per cent for his first three months. That five-point boost would be enough to deprive Labour of its majority, all other things being equal. But they are not equal. Gordon Brown will take over, and we know that he has staying power. The question is whether David Cameron has it too.

On the basis of this week's shrill and silly performance, the Tory leader is in danger of running out of puff. He needs to calm down and take it steadily and seriously. As he said, about 100 days ago, to the Labour Chief Whip: "Now. Have you finished?"

j.rentoul@independent.co.uk

The writer is chief political commentator for 'The Independent on Sunday'