Not since Sir Geoffrey Howe resigned from Margaret Thatcher's government in November 1990 and set in train the events that led to her removal from Downing Street has a Cabinet departure been calculated to cause so much damage. Hazel Blears may be a smaller political beast, in every sense; but the timing of her announcement, just an hour before Prime Minister's Question Time was devastating, and was intended to be so.
It was even more extraordinary that it was done on the eve of crucial elections. The unwritten political convention is that with a poll looming, unity is all; personal differences are put aside for the good of the party. To cause such damage, just a day after it became clear that Jacqui Smith, the Home Secretary, would be standing down, was almost wilful.
Blears must have been very angry indeed, as judging by her resignation letter, she was - though not over Brown's handling of the government, but the way he had failed to support her in the expenses controversy.
How can he go on, everyone asks? How can Gordon Brown continue to sit in his office while his Cabinet ministers head out the door? "This government is collapsing before our eyes," David Cameron taunted in the Commons on Tuesday, and that is certainly how it looks. But prime ministers can be very resilient creatures. They hold most of the cards in the constitutional pack, the strongest being the power to reshuffle, as Brown did on Friday.
Don't forget that, in 1995, it was all over for John Major. The Tories were trailing Labour by 20, even 30, points in the opinion polls and had been reduced to the control of just one county council in the local elections that May. Major was in an even worse position than Brown because he effectively had no majority in parliament; a number of Tory Euro-rebels had been stripped of the party whip.
He was toast and everyone knew it. But he resigned as Tory leader, fought an election that only John Redwood among his Cabinet colleagues was prepared to contest, and won. He had another two years in office before sinking to the inevitable landslide defeat in 1997.
The point is that prime ministers can be in the deepest mire and still extricate themselves because they have all the ladders. If Brown acts swiftly and decisively enough, he will give himself a breathing space. It may not do him any good in the long run and it may not stave off defeat for Labour at the next general election. But it could keep him in office for another year.
However, while this might serve Brown's purpose to remain at No 10 for as long as he can - and that is, after all, what all prime ministers seek to do - there is a difference between this government crisis and those that have gone before: it coincides with two other crises - one that is engulfing parliament as a whole and another that continues to grip the economy.
As Anthony King, constitutional historian and professor of government at Essex university, puts it: "No one alive can remember a crisis quite like this one. Prime ministers have been in deep trouble before - for instance, Sir Anthony Eden in 1956, and John Major a decade ago - but they have never been in such trouble in the midst of an economic crisis and when the legitimacy of parliament itself was called in question. There is also no precedent since the 19th century for a government and a governing party disintegrating so publicly.
"However, these events hardly raise constitutional issues. Dozens of prime ministers have taken office without their own electoral mandates - Churchill in 1940, Macmillan in 1957, Callaghan in 1976, Major in 1990 - without anyone questioning their authority, and ministers can resign whenever they like and for whatever reasons they like."
Newspaper disclosures over the past four weeks of the way many MPs have routinely abused the allowances system have, nevertheless, caused popular disenchantment with the entire political system. At the same time, the recession is costing people their jobs and their homes. In such a concatenation of circumstances, to have a prime minister who does not appear to command the support even of his own Cabinet is dangerous.
There is paralysis in Whitehall in a way there was not in the last two years of the Major government, which Labour likes to cite as the low watermark of governmental competence. Britain's reputation overseas is being harmed and business leaders trying to sell into foreign markets are exasperated by the impression such dithering gives abroad.
Legislation that will have a significant impact on people's lives continues to trundle through a shell-shocked parliament, without being properly scrutinised. On Monday, Blears was in the Commons championing her department's latest measure, the Local government, Economic Democracy and Construction Bill. As she spoke, Andrew Mackinlay, an exasperated Labour backbencher, rose to his feet: "All this about economic prosperity boards tying up with some other bodies is gobbledegook," he said. "Most people have had enough of all this... I say to her that this is complete nonsense and that we have had enough..."
As Mackinlay said: Britons have had enough of all this. And yet we are supposed to wait for more when a Brown administration brings forward another legislative programme in the autumn. Even the Guardian, often a cheerleader for Brown, has had enough. In a leading article on Tuesday, it said: "The truth is there is no vision from him, no plan, no argument for the future and no support. The public sees it. His party sees it. The Cabinet must see it too."
Which brings us to the nub of the matter. It is said that rebel MPs are circulating a 'Gordon must go' letter; but they did that last year and it didn't do them much good. The only way a prime minister can be forced out is if he loses a vote of confidence in the Commons.
That is the danger that faces Brown today. As one Labour MP put it on Tuesday, he has a "window of opportunity" to reassert his authority with a brutal reshuffle that shores up his personal position. But it would do the rest of us a favour if they simply pushed him out of it.
