This has been the subject of considerable controversy in Massachusetts. Some students have demonstrated against Professor Weitzman, insisting that he publicly apologise for his supposed behaviour. They have been supported by an organisation called Manure Movers of America (I kid you not). The most mysterious element in this strange tale, however, is why the professor might have wanted a vast amount of fertiliser in the first place. Personally, I have long had my suspicions about where economists acquire the basic material for their arguments.

British election campaigns are not dissimilar to this episode. Intelligent and otherwise reasonable politicians will do almost anything to acquire muck to chuck at their opponents. They know it is the wrong thing to do, but seemingly cannot help themselves. Much like Professor Weitzman’s suggested secret trips to the farmyard, it becomes a bizarre form of compulsion.

So in the past week we have witnessed the Labour Party implying that a Conservative triumph would ensure the economy reverts to a state last seen in the Dark Ages and that every school and hospital in the land would be sold off to the highest bidder. Not to be outdone, Michael Howard has hinted that ministers are virtually assisting those who commit violent crime and that by the end of a third Labour term about 70 per cent of the population will have come here from Somalia. The Liberal Democrats, meanwhile, condemn this strife — while intimating that Tony Blair has been a hired killer in Iraq and Mr Howard enjoys burning crosses in his spare time. There is no vehicle in the world large enough to transport what is being hurled on the hustings.

There has been, though, one aspect of these exchanges that is entirely novel. The most implausible statement uttered by a politician since the contest began almost a fortnight ago was made about himself and not one of his rivals. The prize for the most incredible pledge of campaign 2005 has already been won by the Prime Minister.

The moment came during the press conference to launch the Labour Party manifesto. Mr Blair repeated his previous assertion that this would be his last election and his final term in office. When pressed by journalists on whether he therefore intended to serve right up to the dissolution of the next Parliament, he replied, to quote precisely: “Yes. When I say a full term, that is exactly what I mean.”

Put all the sound and fury of political battle to one side for an instance. If this is “exactly what I mean”, then it is almost laughably ludicrous. If the Prime Minister sticks to his word, there would be a four, possible five-year struggle between senior members of the Cabinet to replace him. This would be fundamentally destabilising for his administration. It would be an appalling spectacle for the country. As 2009 or 2010 drew closer, Mr Blair would begin to mutate from a lame duck to a dead one. Government in Britain is not possible in these circumstances. It would court disaster. It will not happen. So confident am I of this that I am willing to put in print now that if Mr Blair is still in 10 Downing Street a few months before the next polling day, I will spend part of that election period walking up and down the streets of his Sedgefield constituency dressed as a pork pie.

Why is he saying it, then? Because he thinks he has to. He feared that the question of “how do you know what you are getting if you back Labour?” would haunt the whole election if he did not. He has, in effect, adopted a position which is mad but useful. It is a device to take him through the next few weeks and to be abandoned afterwards.

It is more interesting to ask why he is not being quizzed harder on this matter. I think that there are three reasons. First, the press and the pundits do not in their hearts believe that mid-term prime ministerial succession is an outrage. Nor is it. Of the past 20 people — including Mr Blair — who became prime minister, 14 of them acquired the post before a general election and not at one. Ten then went on to win a mandate of their own. Four (Balfour, Chamberlain, Douglas Home, Callaghan) did not.

Secondly, most observers think Mr Blair would be round the twist to want to remain Prime Minister for as long as 12 or 13 years. Finally, the polling evidence is that not only do the public expect him to quit earlier than he claims, they also want him to. An ICM poll published yesterday, for example, revealed that by a margin of 56 to 38 points, voters expect that Mr Blair would step down in the middle of the next Parliament. Another, by Communication Research , demonstrated a consensus in favour of Gordon Brown moving from the Treasury into Downing Street sooner and not later.

This does leave us in a paradoxical situation. An election which is being fought in part on the notion of “trust” has the Big Fib at the centre of it. It is an untruth which the electors as much as the elected are complicit in.

Mr Blair’s is talking what it is reported that Professor Weitzman has been taking. And we are buying it from him.

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