I am one of those who admire David Cameron as Leader of the Opposition. I find his position as a liberal Conservative sympathetic; in this he is not unlike the Republican John McCain, who would be my choice in the American presidential race. Cameron has the gift of rising to the occasion in his big speeches. He takes his decisions quickly and rationally. He has a steely quality that he will need if he becomes Prime Minister.

All of this is fine and, indeed, quite impressive. Of course, there are policy issues on which I think he is mistaken. I regard academic selection as essential to providing the opportunities for gifted students; selection should not be confined to private education, which has been given an inordinate advantage.

There is, however, one Conservative commitment that makes no sense, and does not match the reality of the world's economic outlook. The Shadow Chancellor, George Osborne, took a leaf out of Tony Blair's book - always a dodgy thing to do. Before the 1997 general election Mr Blair promised that Labour would follow Conservative spending plans, in order to protect his party against the suspicion that it would be recklessly extravagant. Mr Osborne has promised to follow Labour spending plans, in order to protect his party against the public's fear that it would cut welfare spending. That is a Conservative commitment, and must have been approved by Mr Cameron himself.

The first difficulty is the time scale. Labour is behind in the opinion polls and one can assume that Gordon Brown will not call an election while he looks like losing it. Therefore the next election could be postponed until the last possible date, May/June 2010. The Parliament that follows could last to the middle of 2015. No one knows what the economic situation will be then. Labour spending plans, formulated in present conditions, may be irrelevant to the circumstances of the next Parliament. This argument alone makes the Osborne commitment a mistake.

However, there is more to it than that. The Government has virtually lost control of the budget balance. It may be necessary to raise taxes substantially in order to prevent the budget deficit escalating. The Government will not raise taxes by £8 billion, though some people think it should. In 2010 whoever wins the general election will probably have to make a choice between higher taxes and cuts in expenditure. The commitment to Labour spending plans could therefore become a commitment to higher taxes to pay for the spending. The Tories can't afford to become the high tax party.

There is also the threat of a serious recession, particularly in the housing market, which itself affects consumer spending and government revenue. I would be pessimistic about the economic prospect in the next two years; I expect there to be a downturn in the US and here. If it does happen, unemployment will rise, government spending will rise, but revenues will fall. In those circumstances, it would be quite ridiculous for a Conservative government to be committed to spending plans that might become wholly inappropriate.

The Government may be making this risk even worse. Alistair Darling, the Chancellor of the Exchequer, has introduced his £30,000 levy on “non-domiciled” foreigners in its least acceptable form at the most unsuitable time. A simple scheme, such as Mr Osborne proposed, might have been widely accepted, but this has in fact been an Inland Revenue scheme liable to cause maximum anxiety. The Revenue is always closing loopholes that are the air vents by which the tax system breathes.

The result is that more than half of a sample of 22,000 wealthy foreign clients say they will either leave Britain or remove some of their assets. The London housing market depends on wealthy foreigners, just as much as Chelsea Football Club. The rest of the housing market usually follows London's lead, up or down.

The City of London, which is Britain's greatest economic asset, depends on the prosperity of global finance. Unfortunately, global finance is still suffering from the credit crunch. It may take several years to reach complete recovery. The City may have to adjust by “downscaling” its own staff. Redundancies will mean that houses have to be sold; if the wealthy foreigners are also moving out, there will be two important groups of London homeowners who want to sell at the same time, but no obvious new buyers. Commercial property is already in recession; houses in London would follow, and the house price recession would eventually spread to the rest of the country.

These are, of course, exactly the circumstances in which the Labour Government would be likely to be turned out. Labour would be blamed, reasonably enough, for the recession, the loss of jobs, the collapse of house prices and the dispossessions. The Conservative Party will need to be free to reconstruct the national finances. Only if we now enjoy a couple of years of stable prosperity would it be prudent to commit to Labour spending plans. Yet if there were to be a couple of years of prosperity under Labour, it is unlikely the Conservatives would win a general election.

The Conservatives have reason to want to defend themselves against accusations that they would cut social expenditure. These allegations have been made by the Labour Party at every general election for years. But the economic situation is already worse than it was when the pledge was first made. In 2010 Britain may still be in a budget crunch. The Conservatives do not want to be tied to Labour mistakes.

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