For some time commentators have been saying that the Conservatives could not win the next general election because they were stuck on 40 per cent in the polls and needed to get to 45 per cent to have the prospect of an overall majority. There was something in this argument. It would require an extraordinary collapse of the Labour Party for the Conservatives to win outright. Yet now the unthinkable has happened. A YouGov poll yesterday in The Sunday Times put Conservative support at 45 per cent, with Labour on 32 and the Liberal Democrats on 14. These are landslide figures.

Since the 2005 general election I have taken YouGov as my gold standard poll, and Colin Rallings and Michael Thrasher's Media Guide to the New Parliamentary Constituencies as the best way of converting share of votes into share of seats. We are obviously still a long way from a general election, but it is seats in the next Parliament that really matter, if only because the morale of a parliamentary party depends on the confidence of MPs that they can hold their seats. At the end of their guide Rallings and Thrasher provide a simple conversion table. If the latest YouGov figures were to occur at a general election, the Conservatives would win 374 seats, Labour 227 and the Lib Dems 19.

The Conservatives would have an overall majority of about 100 and a majority over Labour of close to 150. Labour would lose about 120 seats and the Lib Dems about 45. This would produce a Conservative landslide comparable with the Labour landslide in 1997, as large in terms of the share of votes, though somewhat smaller in terms of seats. On any result approaching the current YouGov poll one would expect the Conservatives to be in power for the next two parliaments.

The most remarkable aspect of this decline in Labour's support is how rapid it has been. If one takes Gordon Brown's personal rating, that has fallen from a favourable 48 per cent in August to 39 in September, to 30 in October, to minus 10 in November, and to minus 26 this month.

That was some honeymoon.

The event that seems to have triggered the crisis in Labour support was Mr Brown's decision not to hold an October election, after having planned for it, raised money for it and allowed the expectation to run away with him. The response of the voters seems to have been that they should not give their confidence to a government that lacked confidence in itself. However, that was only the trigger; there have since been a series of administrative blunders that have made the Government look incompetent or sleazy.

Most damaging has been the coincidence of the political cycle and the economic. At the time of Labour's 1997 landslide, I remember writing an article discussing whether this would be a two or a three-Parliament defeat for the Conservatives. In fact, they have found it has been a three-Parliament problem, and they have needed four leaders to reach this point. Anyone who predicted in 1997 would be in serious trouble in the second half of their third Parliament — if they reached that stage — was likely to prove correct. Both Margaret Thatcher and Tony Blair, who were brilliant at winning elections, found their third Parliament was a Parliament too far.

Mr Brown would have found the third Labour Parliament difficult in any case, but he might have survived it and even won a fourth general election. It is the economic cycle that may now be doing the irreparable damage. More than half the voters are now fairly worried or very worried about the threat of a recession next year. They are right to be worried. Northern Rock and the pictures of people queueing to take their money out have damaged the Government's reputation for financial competence; that had been Mr Brown's greatest asset. Northern Rock is only part of a global crisis.

The machinery of this crisis may be hard for most people to understand. Clearly some very senior bankers have failed to understand it. Yet the consequences are clear enough. Six months ago banks had a great deal of money to lend and were more afraid of losing their share of the loans market than they were of suffering a shortage of cash in their own balance sheets. Since early August banks have been reluctant even to lend to each other, and absolutely unwilling to lend on sub-prime risks of any kind.

The world has moved from a period of easy credit to a period of tight credit: from a period when cash is on tap to a period when cash is on top. This has happened before, and will happen again, but periods of tight money are inevitably periods of suffering in business, in housing, in jobs and therefore in politics.

Some people in the Labour Party comfort themselves because public opinion is volatile, and hope it will soon recover. Yet it seems equally likely that the momentum against Labour is still gathering pace.

This no longer looks like a government that had a grip on the nation's problems. The Conservative Shadow team, Cameron, Hague, Osborne, Fox, Davis, is younger, except for Davis, brighter and more self-confident than the Labour Cabinet, harassed by events. If the credit crunch continues, debts, defaults and repossessions will mount. A family that has lost a house or a job will not vote Labour in 2010. It is no wonder that the morale of many Labour MPs has collapsed along with the polls.

In a parliamentary system, where the Government has to have a majority in the House of Commons, one cannot have a two-term limit, like the US presidency. Yet Labour's third Parliament is already looking excessively tired, and May 2010 seems a long way away. Britain has big problems to solve and an increasingly exhausted government is unlikely to solve them.

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