One should always hesitate before using the "C" word. Political crises occur much less often than political commentators would have their readers believe. But this weekend, a crisis began.
It is impossible to foresee the outcome. There is a fair chance that Gordon Brown will no longer be Chancellor by the end of the month, in which case Mr Blair would face a leadership challenge immediately after the election, even if he won. If Mr Brown does remain Chancellor until the election, and Labour wins, it is now almost certain that he would begin the next Parliament on the back benches.
"And Labour wins": suddenly, there is a doubt. About 18 months ago, I described the Blair/Brown relationship as a San Andreas fault: the one major uncertainty which could disrupt all electoral calculations. It now has.
This weekend, partisans in both camps were considering their options. Should Gordon resign? Should Tony fire him? The PM had already decided that Mr Brown would have a much less important part in this election campaign. Alan Milburn will be in charge of the manifesto, not Gordon Brown. Mr Brown will chair the London press conferences, but their importance will be greatly diminished, because the PM will rarely be there.
Tony Blair has decided to take this election to the people. He will be visiting nursery schools and tsunami detection units, to turn those into the day's news story: campaigning by photo-opportunity. In the meantime, John Prescott will be addressing miners' galas and punching egg-throwers, while Gordon Brown will have his press conferences. The PM's advisers would not care whether anyone turned up.
They would be content if the Six O'Clock News ran as follows. Items 1-8 shared by Messrs Blair and Howard, Tony having the better pictures and the better poll results. Items 9-10, Charlie Kennedy. Item 11with background picture of a Japanese journalist earnestly taking notes: "The Chancellor today stressed the importance of endogenous neo-classical growth theory."
Gordon Brown was put in charge of the press conferences solely as a sop to self-importance.The Blairites must realise that this was a serious miscalculation. Gordon Brown's press conferences will overflow with journalists eager to drive wedges between him and the PM. Mr Brown will inevitably provide material. The problem will be compounded if Mr Blair is up country.
This is a principal anxiety in any election campaign, for all sides. The PM/leader is in Derby. The Chancellor, or shadow Chancellor, appears to have gone off message in Dundee. At London HQ they are frantically trying to find out what exactly has been said, while devising a form of words which will enable them to make the straight-faced claim that everyone is in entire agreement about everything.
Now there is a Chancellor who does not give a damn whether he stays on the PM's message; he prefers his own. Labour spin doctors will be desperate to convince - and coerce - the electronic media: "Forget this Brown/Blair row rubbish. Tony patting tiny tots' heads - that's the story." It is unlikely to work. The Blairites' worst nightmare could come true. They will lose control of the news agenda, and during an election campaign.
Given all that, the real question is not whether Tony Blair dares risk sacking the Chancellor. It is whether he dares risk keeping him.
If Mr Brown does stay, it will be almost impossible for the Government to give a clear exposition of its principal policies. On both public service reform and Europe, there will be a Blair agenda and a Brown one. Voters tend to punish divided parties. If the split becomes the story, Labour could lose.
Journalists are often accused, by no means always unfairly, of paying too much attention to personalities and too little to issues. This time, however, personalities are the issue. Ten years ago, Messrs Blair and Brown rarely disagreed. Though Mr Brown had a much greater emotional attachment to the old Labour Party, he, too, understood that there had to be changes. Europe was Mr Blair's main political outlet for his emotions. On that subject, Gordon Brown was a lot less intense. But he was still a good European.
In those days, the two men would have taken an hour to draft common statements on Europe and the public services. Ten years later, personalities make that impossible. During those 10 years, Mr Brown has conducted the third longest sulk in British political history. He will never overtake his two rivals, Rosebery and Ted Heath; they both managed to sulk for three decades. But Mr Brown can console himself. The other two sulked from the sidelines; he, from the centre. They were rapidly marginalised; his gloomy sullenness overshadows the entire Government.
Mr Blair made his big mistake in 1994. Two years earlier, Mr Brown had been the dominant figure in Labour's early forties generation. But during those two years, he had been overtaken, by the man who used to refer to himself as Gordon Brown's younger brother. When John Smith died, Tony Blair seemed the obvious replacement to a significant majority of those who were about to elect the new leader.
Unlike Mr Brown, Mr Blair was aware of this, which helps to explain his error. When he persuaded Gordon Brown not to stand against him, he wanted to protect his older brother from the humiliation of a heavy defeat. But this only enabled Mr Brown to create his "stab in the back" myth. With increasing passion as the years go by, Gordon Brown has convinced himself that he was cheated out of the Labour leadership by a letter from Peter Mandelson and a dinner in Islington.
It would have been better for Tony Blair if there had been a contest. He would have won easily, and thereafter the Brownites could not have been able to claim that the Blair leadership lacked legitimacy. Mr Brown himself would have been less unmanageable.
Not that he lacks legitimate grievances. In November 2003, Tony Blair lost his nerve and decided that he could not stay in Number 10 much longer. He asked for Gordon Brown's help to negotiate the final few months and promised to try to pass on the premiership. Gordon agreed. Some months passed, and Mr Blair recovered. He also decided that a Brown premiership was not such a good idea. So he reneged, with the excuse that Mr Brown had not given him enough help. Gordon Brown is entitled to be outraged at being fobbed off with such transparent mendacity.
Indeed, one reason why Mr Brown is now so angry is the damage his self-esteem has suffered, because he has often been successfully deceived by a man whom he regards as by some way his intellectual inferior. Forgive Tony Blair? Gordon Brown is finding it hard to forgive himself.
The Brown/Blair conflict has flared up before, only to subside. It is impossible to see how that could happen this time. The two men's relationship is now, at last, finally dead. The only question is whether the Government can survive.