Dominic Lawson: It wasn't Gordon Brown who assassinated the Prime Minister. It was David Cameron

It was that voice again: the one with the contrived hesitations and the catch in the throat. Yes, Tony Blair had decided to broadcast his statement on his future as Prime Minister in the same key as his televised remarks about the death of the Princess of Wales nine years ago. That performance marked the high point of Blair's popularity. While senior Labour party politicians found his quavering tone emetic, the British public fell for it.

Unfortunately for the Prime Minister, the British public will not empathise so tearfully with him at the prospect of his own political death. Coincidentally, it was also announced yesterday that the official inquest into the death of Diana would begin next year (a mere decade after the event).

The inquest to decide who finished off Tony Blair has already opened. Witnesses to the event seem united as to the malign force that brought about this political car crash. It was Gordon Brown. The Prime Minister's die-hard supporters - and those are the only ones left - utter words such as "coup" and "betrayal". It is not as barmy as Mohamed Fayed's claim that Diana was murdered on the instructions of the Duke of Edinburgh, but it is similarly untrue.

Tony Blair's assassin is a man by the name of David Cameron. He has not been mentioned at all in this connection, although he happens to be the leader of Her Majesty's Opposition.

Mr Cameron has form as an assassin of party leaders. His first victim was Charles Kennedy. Within a fortnight of being elected to the leadership of the Conservative Party, Cameron made a speech in which he declared, "I say to Liberal Democrats everywhere: join me in my mission to put Green politics at the top of the national and international agenda. We're on the same side now... I want our troops in Iraq coming home as quickly as possible."

Those remarks, combined with opinion polls showing some movement away from the Lib Dems towards the revived Tories, caused a most comical panic among Mr Kennedy's parliamentary colleagues. Within three more weeks Charles Kennedy had been forced to quit. The ostensible reason - that he was an alcoholic - would not convince any seasoned political coroner.

Kennedy had been drunk for years. His colleagues put up with it because his alcohol-fuelled charm had brought great electoral gains - when the only alternative opposition to Labour was led by men lacking popular appeal to floating voters. Cameron capsized that complacent calculation.

Now let us examine the events leading up to yesterday's announcement by Tony Blair. There weren't any: at least, not in the conventional sense of the word. There was, however, a circumstance. That circumstance was the publication just over a week ago of a couple of opinion polls showing that the Conservative Party had the support of 40 per cent of those inclined to vote at the next general election.

Given the margin of error in such polls, the difference between 39 per cent and 40 per cent is meaningless psephologically. Psychologically, however, it is freighted with meaning. The figure of 40 per cent is one which - if actually manifested in a real vote - would result in the Conservatives gaining an overall majority in the House of Commons.

Much has been spoken about parallels with the defenestration of Margaret Thatcher by the Tory party. It is true that the Conservatives were far behind Labour in the opinion polls at the time. But there was absolutely nothing new in that. As one very experienced Tory MP remarked to me yesterday, "We were almost always behind in the polls under Margaret - except during those strange periods called general elections."

In this context it is interesting to note what a high proportion of the Labour MPs writing open letters urging Mr Blair to quit now "for the sake of the party" are among the more recent intakes. These are men - they do all seem to be men - who have had a quite unnatural political career: throughout their time in the Commons they had never endured the experience of seeing their party lagging badly in the public opinion polls. They have all the brittleness of people completely unused to normal political adversity.

Of course, their nerves will feel all the more brittle if their parliamentary majorities are below, say 8,500. Observe that of the seven Parliamentary Private Secretaries who resigned on Wednesday, only one had a majority above that figure. I believe it is a medical fact that sudden, involuntary bowel movements can cause equally sudden lapses in judgement. These men, said John Burton, the Labour party agent in Mr Blair's Sedgfield constituency "owe their own election to Tony Blair, and they are more likely to lose their seats under a new leader." Most ridiculously of all, Mr Blair was never going to be their leader at the next general election in any case.

When the Tories conspired to force Mrs Thatcher out of 10 Downing Street it was against the background of a seismic political event - the public's noisy, and at times violent, rejection of the poll tax.

A large number of Mr Blair's Labour opponents say that there has been a similar breaking point for them - their leader's decision a few weeks ago not to back "an immediate ceasefire in Lebanon". Indeed, on the BBC's World at One yesterday, the chairman of the Islington Labour Party gave this as his reason for backing the calls for Mr Blair's removal from office.

This shows, as if it needed to be demonstrated, how very untypical of the country as a whole Islington is - or perhaps how very unrepresentative of the country Labour activists are. For the average voter - and I include in that category the average Labour voter - this issue is as far removed from their daily concerns as the civil war in Darfur.

I do understand that Mr Blair's unhesitating endorsement of Mr Bush's line over the conflict in Lebanon was a poisonous echo of his uncritical support of the US invasion of Iraq, and the profound loss of public trust bound up in that; but it would have made no difference even to events on the ground in Lebanon if Mr Blair had joined with the Arab League in calling for an immediate ceasefire.

As for the idea that it would have meant a difference to the vast majority of people in this country - as poll tax bills thumping on doormats certainly did -well, that is a proposition that could be seriously entertained only by political activists and the writers of newspaper editorials.

It was interesting that in his brief statement yesterday, Mr Blair pointedly remarked that while the Labour Party was in tumult over his leadership plans, "We've got the blockade on the Lebanon lifted today." Which, translated, means: there is the real world which carries on regardless of the neurotic agitations within the Labour Party.

Meanwhile Mr Cameron chuckles to himself, possibly while stroking a white cat...