Bruce Anderson: Brown's task will be to distance himself from Blair's legacy - Cameron's job is to stop him

Brown will hope to persuade voters that everything which they most disliked about Blair is buried with him

Let us pause to consider an extraordinary phenomenon. The destruction of Tony Blair is without precedent in British politics. Normally, prime ministers either choose their own moment of departure or are defeated at the polls. Traditionally, the Labour Party is generous. Attlee and Callaghan lost elections, but then held on until they were ready to go. It is often asserted that unlike Labour, the Tory party is ruthless towards its leaders, but this is a myth. Heath lost two elections before he was pushed out; Balfour three, and he did not fight to stay on. Nor did Eden.

Mrs Thatcher was an exception, as usual. But if her re-election campaign had been run with even a minimum of competence, she would have earned the right to carry on. Moreover, her crisis came three and a half years into a parliament, when the electoral shadows were lengthening.

A year ago, Tony Blair won his third term with a majority that John Major would have despairingly envied. Labour could lose 20 by-elections in this parliament and still be better-placed than Mr Major was in 1992. Certainly, the local elections were bad for Labour and good for the Tories, but not so spectacularly good that any Labour MP need despair of the outcome in 2009. Governments traditionally suffer in mid- term contests.

That brings us to one of Mr Blair's problems, and there is a parallel with John Major's early days. Our PM is not used to political adversity. By the time that he became Labour leader in 1994, the Tory party was destroying itself with such enthusiasm as to do the Opposition out of a job. Massive majorities ensued. It all seemed so easy. So when the seas turned rough, Mr Blair was wholly unprepared.

That is unusual. At least since Disraeli, all our prime ministers had either been hardened in the fire of contentious opposition or had undergone a difficult ministerial apprenticeship - or both. The one exception was John Major. This added to his sufferings, as it now adds to Mr Blair's.

There is one prime minister whose predicament could be compared to Tony Blair's: Lloyd George. By 1922, he was at the head of a nominally coalition government which was in practice a Tory one. He had little affinity with the average Tory MP, and was widely distrusted by his supposed back-bench supporters. Suddenly, he was gone.

In Tony Blair's case, there is no suddenness, but at last, after 12 years, the Labour Party's rejection mechanisms are piling into action. There is a simple explanation: the Iraq war. Earlier, while Mr Blair won elections and stuffed the Tories, the Labour Party was prepared to forgive him almost anything. Iraq broke the bonds.

Over the next few years, there will be a score of books and doctorates on the end of the Blair premiership. In the interim, anyone interested in politics should sit back in the theatre seat for an enthralling spectacle.

But not quite anyone. David Cameron has no such luxury. He will be too embattled. Yet there is one luxury which he does enjoy: luck.

That used to be true of Margaret Thatcher. Jim Callaghan's refusal to call an election in October 1978; Michael Foot defeating Denis Healey; Galtieri rejecting the Peruvian peace proposals; Scargill rejecting Nacod's peace terms: today, these events may seem to be the arid acme of dry-as-dust political arcana, but at the time they determined the course of history. They all went right for Margaret Thatcher, and by God she exploited the opportunities this gave her. If she were a batsman and had been dropped early in the innings, a great groan would have gone up from her opponents' supporters. They would know that by the time she gave another chance, she would have scored 200. To switch sporting metaphors, offer her a glimmer of luck and she would punch an armoured division through it.

She then ran out of luck, and poor John Major never ran into it. That had to wait for Tony Blair. Back in 1997, the Cabinet Secretary, Robin Butler, said that we now had a lucky prime minister for the first time since Margaret Thatcher. But, just as she did, he has run out of luck.

Yet there is a crucial difference between Tony Blair and Margaret Thatcher. She used her luck to add to her political momentum and get things done. He just luxuriated in his. David Cameron is determined to follow Lady Thatcher's example. He knows that luck only gets you so far in politics, and that eventually, success has to be earned.

One point about Mr Cameron ought now to be clear to the most idle, ill-informed and malicious observer. The new Tory leader leaves nothing to chance. Agree with him or disagree, there is always a strategy. He and the people around him never stop thinking about how to press home advantages and minimise disadvantages.

Very early on, the Cameronians decided to divide their forces. As long as Tony Blair was in No. 10, he had to be a focal point of attack. He would be the gladiatorial opponent in the House of Commons. But not at the next election. Hence the second thrust, launched against Gordon Brown. David Cameron was determined to get his retaliation in before Mr Brown had even come on to the pitch.

That task is becoming steadily more urgent. Mr Cameron is aware that as soon as Mr Brown becomes leader, he will try to distance himself from the political and moral failings of the Blair regime. The new Labour leader will hope to persuade the voters that everything which they most disliked about Tony Blair is buried with him.

So the Tories' task is to undermine Gordon Brown's claims to moral authority. This should not be impossible. Mr Brown has been accused of lying over the Bernie Ecclestone affair. He is also the man who has stolen pensioners' funds on a scale which makes the late Robert Maxwell seem like the most upright Scottish actuary. Mr Brown's assertions that he is an honest man who is solely concerned to uphold the public interest could be easily be contradicted by several million people who have seen the destruction of their hopes for a prosperous old age.

Mr Brown is one of the few people in British politics who still under-rates David Cameron. He cannot wait to get stuck in, and is confident of an easy victory. David Cameron does not do advance confidence in victories. He waits until they are won before he celebrates. But in a realistic spirit, and without in any way under-estimating his next opponent, David Cameron is looking forward to becoming the senior party leader and taking on Gordon Brown.

Both sides are ready to commence battle. There is only one difficulty; a plaintive, absurd little figure who will not get off the battlefield. Tony Blair, who used to dominate that field, cannot seem to realise that his failure to vacate it merely piles indignity on futility.