Bruce Anderson: Mr Cameron has chosen his moment to call on the Prime Minister to go, and go now

There was a noteworthy incident last week. At Prime Minister's Questions, David Cameron called on Tony Blair to resign. That might sound the stalest of stale bands. Throughout modern history, Opposition leaders have been inviting Prime Ministers to resign and receiving crisp responses. That was part of the background noise of politics.

Not this time. Mr Cameron's comments earned headlines and still resonated at the weekend. This was not just a successful hit at Tony Blair. It was a vindication of the Cameron strategy.

When he became Leader, Mr Cameron announced that he would eschew reflex, cliche-ridden, adversarial politics. This made some Tories feel uneasy. Most politicians enjoy ding-dong politics, particularly when easy targets are on offer. After a dozen years at the receiving end of adversity, most Tories wanted revenge. Some of them were baffled that their leader did not share their instincts. This sounded as improbable as a vegan in a rugger scrum.

Yet David Cameron was determined to hold his fire, for sound tactical reasons. Though MPs might relish a Punch and Judy show, especially when they have the truncheon, the voters would switch off, both metaphorically and literally. The cruder forms of political warfare were merely politicians' self-indulgence.

On the basis of last week, Mr Cameron had a point. He was adversarial; he received a hearing. He intends to go on doing so, which means a subtler approach than might seem necessary, given Mr Blair's terminal political illness. But the Tory leader wants to reflect the public mood. Though disillusion increasingly shades into disgust in many quarters, there is still a lot more in sorrow than in anger. A great number of voters may feel let down; they are still not ready to jeer. Aware of this, Mr Cameron will not go too far.

The Tories have a further problem. There is a cruel phrase, ideally applicable to Mr Blair's degringolade. But it has been used already, in a context which prevents the Tories from re-using it. It was Norman Lamont's description of John Major's government: "In office, not in power.''

Given recent developments, there could be a variant on that phrase. In conversations last week, a couple of people whose assessments are to be respected were confident that there would be no criminal charges over cash and peerages. One of them said that No. 10 should be grateful to the police. Their unsparing behaviour will lend credibility to the absence of prosecutions. The police will be able to insist that although they shook all the trees, nothing fell out. The public might believe them.

Towards the weekend, however, it seemed less certain that Mr Blair was out of danger. Suddenly there was a different balance between rumour and counter-rumour; a contest which will continue until Commander Yates makes his report. But there is one curious development. Mr Blair's acolytes insist on his innocence. Yet they will not use the obvious slogan for the May elections: "In office, not in jail''.

It is impossible to withhold one's sympathy from Mr Blair's close associates. Most of them are able and honourable. They could all claim that they deserved a more dignified conclusion to their political careers. Never in their wildest nightmares did they think that the Blair years would end like this. Tony Blair's plans for his final few months had tawdry aspects, such as triumphal tours and appearances on children's TV programmes. But that is preferable to interviews with the police. Better Blue Peter than flashing blue lights.

Even if there are no indictments, some of the PM's advisors will bear the scars of all this for the rest of their lives. Yet Mr Blair himself seems immune. He does share one characteristic with Margaret Thatcher: a creative schizophrenia.

During her Premiership, she was frequently discontented - with the government of which she was the head. When right-wing industrialists tentatively suggested that perhaps her Ministers could do more to advance the free market, she would often astonish them by the vehemence of her agreement. Her political sense of self never depended on her government's performance.

The same is true of Tony Blair. He had the right ideas. What a pity that he could never find the ministers to implement them. Solely concerned with mankind's future, he lives on a higher moral plane than other politicians. How could anyone suggest that he might be sleazy? After the triumphant exit in which his critics are confounded and his legacy is secured, no wonder he is looking forward to spending a good deal of time in the United States. There, they will take him at his own valuation.

When that happens, Gordon Brown will be delighted, just as long as there are no pictures on British television. Over the past few months, there have been two interesting silences. The first was predictable: Gordon Brown's, whenever the Prime Minister was in difficulties. In such circumstances, Mr Blair knows that the Chancellor will always be at his side, smirking.

But Mr Brown has been equally reticent about his own plans. Silence does not mean absence. Whatever Gordon Brown's vices, they do not include idleness or lack of ambition. Night and day, he is planning his Premiership and he is not intending to consolidate. Electorally, that might not be a bad idea. If Mr Brown were to announce that the vast number of reforms which have been introduced over the past 10 years needed time to come to fruition and that he had no intention of replanting the garden, the voters might be reassured. But he will not be saying that - because he does.

There are a couple of difficulties - money and inflation. The Chancellor has already spent too much money and the Bank of England is worried that he is not the only one. Between now and the election, there seems to be no way of avoiding financial stringency and the threat of higher interest rates. Mr Brown's mood will not be lightened by the thought that Tony Blair was able to enjoy the good economic years while he is left with the downturn. It is not easy to see how he could escape from the constraints. But he will try.

Just after the Tory Party conference, David Cameron re-deployed some of his advisors. He told them to start planning Gordon Brown's Premiership. Since then, they have been hard at work, but they are still not ready to reveal their conclusions. As one of them put it: "It would be silly of us to give him ideas which he might not have thought of.''

As the local elections approach, we can expect the Tories to come up with pre-emptive attacks. If he has to wait in silence while the PM wallows in embarrassment, Mr Brown's temper will not improve. As the year draws on, we will see how successful the Tories have been in predicting Mr Brown's moves. We can confidently predict a ferocious battle, with plenty of adversarial politics.