Gordon Brown should not forget the basics. As Chancellor, he has always liked to range more widely than the traditional Treasury brief, covering corporate governance, housing and planning and skills. And as the long-awaited date of his expected takeover as prime minister approaches, so his interests have broadened further to include terrorism, climate change etc.

But his prospects as prime minister, and particularly his chances of winning a fourth term in his own right, still depend on the familiar influences of real incomes, inflation and unemployment. He would never be so crude as to say “it’s the economy stupid”, the slogan of the 1992 Clinton campaign. But the economy remains the necessary, if not the sufficient, condition for further Labour election victories.

Indeed, as David Cameron recognises, the strength of the economy until now has been the main reason the Tories are not doing better in the polls. Labour’s rating has been supported by the influx of Polish and Central European migrant workers who are helping to hold down wage and price levels here and to boost growth, as well as by imports from China and India.

Real incomes are now starting to be squeezed. But imagine if the economy really turned sour and/or interest rates rose sharply. That could result in severe problems, given the high levels of personal debt.

The benign economic background since the mid-1990s is not inherently destined to continue. There are risks. Unemployment is already rising and the Bank of England has been anguished about inflation trends. Any downturn could be fatal for Mr Brown’s political and electoral ambitions.

Moreover, the comprehensive spending review this winter will limit the scope for new government initiatives for the rest of this Parliament. The growth of public spending will slow from a rate of more than 5 per cent a year up to 2005 down to 2 per cent at most from 2008. As George Osborne points out in his interview on these pages, the public finances are already very tight. There is no scope to relax borrowing: far from it.

Most worrying of all for Mr Brown, new figures from the British Social Attitudes survey show that support for raising taxes and spending is now lower than at any time since Margaret Thatcher’s heyday in the mid-1980s. Just 46 per cent think that taxes and spending on health, education and social benefits should rise, whereas 50 per cent say that they should stay the same as now, or be cut. In contrast, as recently as 2002, 63 per cent favoured higher spending and taxes.

We are likely to hear none of this at lunchtime today. Since Mr Brown downgraded his short-term forecasts, his update in the Pre-Budget Report should look pretty favourable, notably for growth, while buoyant tax receipts have limited the deterioration in public borrowing. So expect some gloating, along with the long-term visionary stuff: Prudence goes Green will be the theme.

What Mr Brown does now as Chancellor will determine what he can do later on as prime minister. Even then, he cannot, will not, ignore the economy. So his successor as chancellor, Alistair Darling or, as a longshot, Jack Straw, can expect Mr Brown to keep a very close eye on the Treasury.

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