Last week the time came for me to leave the House of Commons for good, more than 20 years after I was first elected. I toured the lobbies shaking hands with policemen, doorkeepers and members. My last trip was to my bare office, its walls stripped of pictures, the files gone and the bookshelves empty. The final task was to close down my computer. Up on to the screen flashed the question: “Are you sure you want to quit?” If my keyboard had a key marked “definitely” I would have hit it. I have planned my departure for more than four years. But I like to think of Tony Blair’s reaction when his computer poses him that query.

He has created an unusual political situation. According to the opinion polls he is headed for a comfortable win. Three working majorities in a row would make him by far the most successful Labour prime minister in history. Yet if Britain re-elects him it will be on his promise not to remain in the post throughout the coming parliament.

As Margaret Thatcher was making her brilliant last-ever speech to the House of Commons as prime minister (in a highly emotional debate after she had announced that she was quitting) a Tory MP yelled out: “Take your resignation back!” Today, I feel like shouting the same at Blair.

His pledge to leave Downing Street was unnecessary, another of his stupid mistakes. It came when he needed heart surgery, and the announcement was meant to divert attention from his health. It is a textbook example of how short-term considerations lead even experienced leaders to commit strategic blunders. Blair has a hundred faults, but with these errors he is going to give the premiership to Gordon Brown, who will be worse.

Last week Blair made another catastrophic miscalculation by saying that it would be daft to move Brown away from the Treasury. It would, of course, be sensible. Brown blocks every reform initiated by Blair. The chancellor denies the first lord of the Treasury any influence on economic policy, and tries to shut him out of health, education and social security too. The poor relationship between the two men threatens the government with paralysis. The only responsible thing for Blair to do would be to move Brown.

If Blair is victorious on May 5 his power is likely to drain away quite quickly. Brown will be seen as the font of future patronage. By promising no change at the exchequer, Blair has extinguished his own hopes that he might do a couple of years’ work unfettered by his pestilent colleague. He has surrendered the opportunity for a new Labour swan song.

Party strategists calculated that unless Blair closed down speculation about Brown’s job, the schism would become the main issue in the campaign and hurt the government’s chances of winning. Wrong. The split is a recognised phenomenon and largely discounted as an electoral factor.

But the decision not to move Brown gives the Tories a new campaign theme: that Blair has pledged to continue the chaos and incoherence that dogs his administration.

Fortunately, Blair’s relationship with the truth is now a distant one. So I still nurture a hope that he may yet renege on this latest promise to Brown. It is encouraging that Brown himself has reportedly said that he would never again believe what Blair says.

Admittedly, even for Blair, the man who told us that the evidence on weapons of mass destruction in Iraq was “extensive, detailed and authoritative”, it would be a challenge to boot Brown out of 11 Downing Street (that is, to do on May 6 what on April 6 he described as “daft”). But maybe the prime minister still has just one more bare-faced betrayal of his chancellor up his sleeve. We must have faith.

The prime minister has capitulated because the Blairites are gripped by an unwarranted pessimism that the Labour campaign will founder without Brown. The chancellor is consistently overrated. Of course his intellect is formidable and his hectoring style brooks no opposition, but whether his performances win votes is another matter.

His campaigning efforts last week were underwhelming. For a man who advocates running on the government’s economic record he seemed reluctant to discuss anything other than the collected works of Nick Herbert, the Tory candidate who has replaced the martyred Howard Flight. Brown would not concede that he had put up taxes nor accept that his 1% National Insurance charge was extra tax. That brand of shifty evasiveness has had its day. The public no longer buys it.

The chancellor’s reputation is based on two excellent decisions: to give independence to the Bank of England and to keep Britain out of the euro. Those two account for Britain’s economic success relative to sclerotic Europe. But his other economic policies are as out of date as his political style. He has interfered and micromanaged. Vast amounts of money have been wasted on unreformed public services. The value of pensions has been wrecked. The UK stock market lingers close to its 1997 level.