Blair spoke from the hip. He was honest and open. It was a disaster

One half of the New Labour brain is still functioning perfectly. The other half seems to have broken down. The strategic half of the brain, that is good at thinking three moves ahead about how to keep the Conservatives down, came up with the clever idea of promising a referendum on the European constitution. But then, last week, the other side of the brain, the half specialising in salesmanship, went haywire.

Thus Tony Blair had one of his worst weeks in the House of Commons, and Michael Howard one of his best. The gains for Labour from the referendum promise may still materialise in the medium term. But that was one of the most inelegantly performed U-turns in recent history. The simple way to see this is in terms of personalities. Gordon Brown is doing what he does best: devising an opportunistic, pre-emptive strike that looks months and years ahead. But Tony Blair has lost his touch - his genius was for presentation and gauging the popular mood.

The decision to promise a referendum on the European constitution looks like classic Brown. It is of a piece with other strategic devices such as the promise not to raise income tax rates and to stick to Tory spending plans for the first two years of the Labour government. In each case, the overriding imperative was to take no risks with Labour votes at the coming election, regardless of possible difficulties down the line.

The referendum is all about taking away from the Tories the refrain, "Give people a say." Indeed, as a result of last week's U-turn, Conservative Central Office had to pulp vast quantities of literature prepared for the European Parliament elections in June. More than that, it is about taking Europe out of the general election campaign. It would have been possible to "Give people a say" on the constitution - assuming it is agreed this summer - by putting it in the manifesto, so that a vote for Labour at the general election would be a vote to ratify it.

But that would depress the Labour vote. By how much is impossible to say, but any unnecessary reduction in the vote is intolerable to Brown. Hence the referendum, which allows the European issue to be safely quarantined in a separate vote which, on its own, is eminently winnable. However, it is too simple to see Brown as the strategist and Blair the salesman. Although their relationship is intensely competitive with regard to which of them should have the top job when, as a political partnership it is harder to tell where Brown ends and Blair begins. It was Blair who insisted on the promise not to raise income tax, jocularly telling Brown's adviser, Ed Balls, to "wash your mouth out" when he suggested, in a pre-election discussion, that they should keep open the option of a higher, top rate of tax.

Last week's U-turn was, therefore, the joint intellectual property of Blair and Brown. Defensive referendums have been, in fact, something of a New Labour device. With little fuss, Blair dropped Labour's unconditional support for Britain adopting the euro that he inherited from John Smith, and then made it conditional on a referendum, which now looks further away than ever. With rather more fuss - a little reminiscent of last week's - he also reversed policy on a Scottish parliament, saying it required a referendum. That U-turn was supposed to be announced in Edinburgh, but it leaked out in my colleague Donald Macintyre's column in The Independent.

If Blair and Brown operate as a joint personality in devising strategy, equally Blair has never operated alone as the New Labour salesman. In the old days, Brown wrote his soundbites, and his advice on presentation and strategy was seamless. There was also Peter Mandelson and Alastair Campbell, who grew in importance in government. But when it came to announcing last week's U-turn, Blair was on his own. Campbell departed last autumn and, although Mandelson has filled a part of that vacuum, he was opposed to the decision and seems to have been unavailable to advise on how it should be handled.

As a result, Blair's announcement on Tuesday, his Prime Minister's Questions on Wednesday and his news conference on Thursday offered a remarkable moving tableau of Blair unspun. He had taken the decision, for the reasons listed in this column last week, but had hardly given a moment's thought to how to sell it. Thus, on Tuesday, he implied a no vote would be re-run like the Irish vote on the Nice Treaty; on Wednesday he said a no vote under the Tories would be worse than a no vote under Labour; and on Thursday he said he didn't know what would happen if the people voted no, but it would be "a very serious thing".

It was open, it was honest, it was from the hip, and it was a disaster. With a 161-seat majority, he seemed to be having his policy dictated by the opposition. A proper politician, the earlier Blair, would have refused to speculate about the consequences of a no vote, saying they intended to win the referendum. That is what Blair finally said, as he struggled to get back "on message" at his news conference, but by then the journalistic herd was stampeding, and was only interested in what would happen if people voted no.

This is not simply a matter of Blair lacking a good candid adviser on presentation. Since the long build-up to the Iraq war, he has become less tentative in his public performance. He is still a politician who makes very few verbal mistakes, but last week he had not given enough thought to the obvious questions he was going to be asked. Maybe this is the growing confidence - or arrogance - of office, or perhaps it is the take-it-or-leave it attitude of a man feeling that he is nearing the end of his time in No 10. Whatever the explanation, Blair's honesty did not help him. People used to complain that he didn't believe in anything, and when he did, over Iraq, he was messianic. Similarly, journalists used to complain about Labour spin, but when they get the unspun Blair they say the Government's position is confused.

The two changes are connected. For years, we have puzzled over his deeper convictions and whether he was simply a supremely effective political robot. Only now, as he approaches his 10th anniversary as Labour leader, are we beginning to get the measure of the man; the longer he is in power the more the guard drops. Also, however, the longer he is in power, the weaker he is. Last week was a measure of that. From the bright, high hopes of winning a referendum on the euro, he is now forced back to trying to win a referendum in order simply to hold the line on Europe and not slip further back.

Conceding a referendum on the European constitution was a sensible way to manage the Europhobic press. In the past, Blair rode that tiger and made it look like strength. Politicians are much weaker than most people think they are, and much of the trick of success is presenting the image of being in control and powerful. Blair has been supremely good at that. What changed last week, apart from the Government's policy on Europe, was that the Blair magic stopped working.