In Hyde Park, they shouted "Blair Out". Might he be forced to take their advice? It is a measure of febrile times that it should even make sense to ask. Hitherto, the occasional spots of bother that have swirled around the most popular Prime Minister in history have had a habit of fading like the morning mist. By-elections never got lost, the Tories never recovered, Blair himself emerged from every scrape toughened and invigorated - the jut of his jaw firmer, the grimace more resolute. Actuarially, he could have gone on like this for another generation, outdistancing opponents and set to beat Gladstone in the political longevity stakes with time to spare.
Suddenly, such a prospect seems a mite less certain. Volatile polls, a monster demo and the unfamiliar sight of cabinet ministers closing ranks like footballers surrounding a fellow player bereft of shorts, have produced a new breed of speculative headlines. Meanwhile, the Prime Minister appears to be having a frantic conversation with himself, producing a new reason for going to war every day, and not so much appealing to audiences for sympathy as engaging in a dialogue with his own conscience.
The most popular man in Britain has become the most isolated, and for the first time people talk about life after Blair. Away from the surveillance cameras, some ask the even more subversive question: how can it be brought about?
The answer is pretty simple. Almost certainly, it can't be - and for reasons largely unconnected with Blair. In theory, the political life of any British prime minister hangs by a thread. In practice, the thread seldom gets broken except by the voters. Indeed, the record shows that chief executives in Britain have nearly as much security of tenure as in the US, where only a bullet or impeachment can dispose of a president before the end of an elective term. When they go between elections, they mostly go of their own accord. Of 24 prime ministerial changes since 1903, 11 have taken place because of a general election defeat, and 13 mid-parliament. But of the 13, most have left because of illness, old age or the understandable desire for a quieter life.
On only four occasions has blood been indisputably shed, in the sense that a premier was forcibly removed against his or her will. Three occurred in the first half of the 20th century. In December 1916, Herbert Asquith had to give way to Lloyd George in a palace coup. In 1922, the Tories ended the coalition in a meeting at the Carlton, pulling the rug from under Lloyd George. In May 1940 (history's most famous ousting), Neville Chamberlain was forced from office by a mass defection of backbenchers following defeat in the Norway campaign. Since the Second World War, just one premier has gone involuntarily - Margaret Thatcher in 1990, the only time that a prime minister has ever been disposed of in "normal" conditions of both peace and single-party rule.
Not only are successful attempts to get rid of prime ministers rare, Tory leaders have been much more vulnerable to what Thatcher called "treachery with a smile on its face" than Labour ones. As well as Chamberlain and Thatcher, Edward Heath (in opposition) was voted out by colleagues, while Anthony Eden and Harold Macmillan might have stayed on but for mounting pressure. By contrast, no Labour prime minister (or even leader, with the arguable exception of George Lansbury) has ever been forced out except at the polls - unless you count the 1979 no- confidence vote that caused Callaghan to go to the country a few months prematurely.
Though Labour leaders have not lacked challenges, they have been singularly adept at weathering them. Prize for coolest customer probably goes to Ramsay MacDonald, who - faced with the revolt of nearly half his cabinet in 1931 - went to the palace ostensibly to resign, and came back still PM, at the head of a Tory- and Liberal-backed national government.
Other leaders were almost as brazen in the face of heavy hints. For the consistently derided and underrated Clement Attlee, seeing off rivals became a way of life. There is the tale, for example, of how - during a particularly gloomy moment in the postwar administration - Sir Stafford Cripps tried to persuade the premier to stand down for the sake of the country. Ernest Bevin would become prime minister, Cripps helpfully explained, and Attlee could have the chancellorship as a sort of consolation prize. Attlee listened attentively, then took his pipe out of his mouth. "No head for figures," he said, and reinserted the pipe. That was the end of the matter.
Other Labour premiers also employed insouciance as a technique. Faced with a revolt over trade union reform in 1969, Harold Wilson gave his carefully crafted answer in a May Day speech at London's Royal Festival Hall. "I know what is going on," he announced, and paused, looking as paranoid as possible. You could hear a pin drop. "I am going on." The hall rocked with applause, and the challenge quickly disintegrated. This was not Wilson's only moment of difficulty - scarcely a month went by during his first term without the revelation of some plot or other, or warnings of plots to come. But none came to much, and in 1976 he announced his retirement at a moment of his own choosing.
Wilson grasped the core secret of survival. To be thrown out, it is not enough to be unpopular. There has to be a rival around whom not just insurgents but the silent majority will rally. He was careful, therefore, only to promote to top positions politicians who had enemies as well as friends. His technique was a form of divide and rule - balancing the competing ambitions and egos of putative crown princes (Roy Jenkins, Tony Crosland, Denis Healey and Jim Callaghan), each of whom cared more about the others not becoming prime minister than he did about getting rid of Wilson.
Blair can draw comfort from the durability of past holders of his office - but in one key respect he is better off than any of them. Wilson's opponents puzzled over the question of how, constitutionally, they could force the issue. They concluded that it could only be done through a meeting of the parliamentary party, though how the matter should be raised at such a meeting, and how the vote should be conducted, was never resolved. Today, because of the electoral college method of electing a leader, the position is even more complicated, and a Labour prime minister even more effectively sand-bagged. A modern Labour prime minister is not exactly elected for life, but he or she comes close. Thatcher was vulnerable because Tory rules allowed for a swift, forensic coup. Under Labour Party rules, involving a process of nomination and voting by party members and trade unions that takes months, she might well have stayed.
In sum, British political history, traditions and structures protect a premier, especially a Labour one, more than is at first apparent and Blair has little reason to lose sleep until the bombs begin to fall. Then the game begins to change. Where war is concerned, history has a habit, not of repeating itself, but of throwing up crackers that make everybody jump.
War is a fissile material, potentially a weapon of mass political destruction. Wars cause anxiety, instability, heightened emotion and the impulse to find scapegoats. Wars have consequences that are unpredictable and sometimes counter-intuitive. Successful wars sometimes lead to khaki election victories (1900, 1918, 1983), but sometimes the opposite (1945). If things go wrong, politicians are called sharply and sometimes unfairly to account. Today, people remember Chamberlain's humiliation in 1940. They forget how close Churchill came two years later to a similar fate, following the fall of Crete. Thatcher became a legend after the Falklands victory; she would have gone abruptly if there had been a defeat. Her eventual departure took place in the excitable mood preceding the first Gulf war in 1990.
War is a catalyst in ways that may be quite unrelated to the casus belli. If war is the continuation of politics by other means, it is also a political accelerator. Ideas that in peacetime take decades to develop crystallise overnight.
Leaders embark on wars expecting to win. We have got used to the phenomenon of the modern war in which the other side takes all the casualties. This may or may not apply in the case of Iraq, but even if it does, it should not be taken for granted that the outcome will simply restore the status quo ante as far as British politics are concerned. Victors expect to determine the postwar agenda. In practice, they seldom can.
There are extreme possibilities. One is that the coming war, with or without the UN, is quick, clinical and clean, minimises collateral damage, changes the regime and reveals a few tons of incriminating anthrax in a palace cellar. The other is that the conflict is protracted, bloody and destructive, destabilising to the region, and the trigger for a campaign of terrorist outrages that heightens western fears, and seems to undermine its point. Possibly the actual outcome will fall somewhere between these poles, leaving loose ends. A messy end could beg many questions. Even a triumph may contain more fragility than appears on the surface: the dismissal of Thatcher, it will be recalled, arose from post-Falklands hubris.
For the moment, there is a curious calm. At the time of Suez, the Conservatives were in crisis - ministers threatened to resign, MPs were deselected, the opposition seemed poised to take over.
There is no equivalent mood today. The public may be on the march, but, for all the rumours of disaffection, the Parliamentary Labour Party (see panel) notably isn't. Far more interesting debates are taking place among Lib Dems and Conservatives. As for the cabinet - there has been the echoing silence of dogs failing to bark. An unpopular war - so what? Labour MPs and ministers alike are trained to be loyal. Anxiously backing the Prime Minister's judgement has become a taken-for-granted habit. Robin Cook and Clare Short may privately grumble but are unlikely to depart. Gordon Brown is notably un-vocal.
Talk of the storming of No 10 is, in sum, wildly premature and wildly unlikely. There is no significant movement to get rid of the premier, no cocked gun, no will for change, nobody who would not appear complicit if things went wrong. Watch out, however, for the most fluid period in British politics since new Labour came to power.
Ben Pimlott is warden of Goldsmiths College