Mr Jackson's defection is less of a liability for the Tories than a bonus for New Labour

Let us be realistic, Michael Howard said in response to the defection of Robert Jackson to the Labour Party, the election is not going to be decided on the basis of what Robert Jackson did. And, of course, in the narrow sense of direct cause and effect, he is right. There was a widespread view that Mr Jackson had long ceased to be much of a Tory. He had been open about his differences with the party and had decided not to stand for re-election; the Tory candidate to replace him in his Wantage constituency was selected months ago. To this extent, Mr Jackson's party allegiance hardly matters one way or the other.

Not standing for re-election because of differences with the party leadership is one thing, however; actually joining the opposition as little as three months before a general election in which Labour could suffer quite substantial losses is something else. In this respect, Mr Jackson's decision to defect while still an MP, his decision to go public when he did, and the arguments he gave to justify his move, could hardly have been more damaging either to the Tory cause in general or to Mr Howard in particular.

Mr Jackson, it should be said, denied that he had deliberately timed his announcement for the eve of the Tories' presentation of key planks of their economic policy. He said he simply did not know that Mr Howard and his Shadow Chancellor were to set out details of their tax and spending review today. If true, though, this statement also speaks volumes about the extent of Mr Jackson's detachment from his party and the negligible efforts made by the party leader and his machine to keep the troops informed and in line. Worse, for Mr Howard, Mr Jackson's defection coincided with a poll forecasting that the Tories could be on course for their poorest showing in a general election for a century.

Potentially most damaging, for the party as for Mr Howard, however, were the reasons Mr Jackson set out in his letter of resignation and subsequent interviews. The extent of public spending cuts being broached by the Tories, he said, was neither possible nor feasible. On the one hand, the cuts would inflict colossal and probably unacceptable damage on social programmes, on the other they would simply prove impossible to implement. He recalled how difficult it had been for the then Tory government to deliver even much smaller cuts in the Eighties. So much for the policies to be presented by Mr Howard and his Shadow Chancellor, Oliver Letwin, with much fanfare today.

Mr Jackson's statements, taken together, amounted to a thinking Tory's catalogue not only of what was wrong with the party under Mr Howard's leadership, but what was right about New Labour. He reckoned that Labour was right about top-up fees for universities, right about Iraq and more right than the Tories in its approach to Europe. As for Mr Blair, Mr Jackson praised him for "rising above narrow sectional interests" and demonstrating "real courage" on both domestic reform and Britain's security. He embraced the attractiveness of his policies to the "middle ground", hailing Labour as closer to the one-nation policies of the traditional Tory party than the Conservative Party of today.

No wonder the Prime Minister pronounced himself "delighted". Here was praise far beyond what he has been accustomed to hearing recently from his own disconsolate backbenchers. Here was, in fact, the inspiration for a whole new set of election slogans infinitely more persuasive than the ones that awkward trio of Milburn, Brown and Prescott have just unveiled.

As a poster for a Blair third term, Mr Jackson's endorsement could hardly be bettered. "It is in this country's best interests that Tony Blair rather than Michael Howard should form the next government," he said of the central reason for his defection. If Tony Blair was searching for positive arguments to boost the defence of his New Labour agenda and divert attention from his clashes with Gordon Brown, Robert Jackson has provided them in spades.