Gordon Brown ought to fire the sad spinners who run Labour by-election campaigns for him. Their campaign in the Crewe & Nantwich by-election has been snobby, smeary and amazingly incompetent, ending up by defining the key by-election issue as being whether Uppingham is, or is not, a “toffs” school. I would myself have put Fettes higher in the toff scale; the voters probably have better issues to consider. Yet the Crewe by-election is itself highly important.

Crewe & Nantwich is - or was - a safe Labour seat. At the 2005 general election Mrs Dunwoody, a deservedly popular constituency member, held the seat for Labour with a majority of more than 7,000. It would take a swing of more than 8 per cent for the Conservatives to win next Thursday's by-election. Yet all the weekend polls suggest Labour will lose to the Conservatives.

A local poll taken for the News of the World gives the Conservatives a lead in Crewe & Nantwich by 45 per cent to 37 per cent for Labour. This constituency poll is supported by two national polls; Yougov in The Sunday Times gives the Conservatives 45 per cent, Labour 25 and the Liberal Democrats 18; ComRes in The Independent on Sunday gives the Conservatives 43 per cent, Labour 26 and the Lib Dems 19.

All of these polls suggest that there would be a Conservative landslide at an early general election. On the YouGov figures, the Conservatives would win about 400 seats and Labour about 200; the Lib Dems would lose seats to the Conservatives in the South but might gain seats from Labour in the North.

In 1997 the electoral tide was running for Labour. Even people who had voted Tory all their lives thought the Conservative Government had lost its way, was wrong about Maastricht, or had been infected by sleaze.

When the results were announced many Conservatives felt relief that their Government had been thrown out of office. Their one regret was the scale of the defeat. With only 165 MPs left, it was obvious that it would take more than one Parliament for the Conservatives to regain power.

The Labour Party had had their year of defeat in 1979. That was a landslide on a smaller scale, but it was followed by bigger Conservative victories in 1983 and 1987, and by a fourth, though less substantial, Conservative victory in 1992. After 1979 Labour was out of office for 18 years, wiping out the whole careers of a generation of potential Labour ministers and MPs.

Most democracies have a somewhat similar electoral cycle. There comes a point at which voters simply decide it is time for a change. I do not much doubt that this point has been reached for the Labour Government.

Tony Blair timed his departure as brilliantly as he timed his arrival. He entered at the moment when a Labour victory had become inevitable, he made his exit within a few weeks of the turning point that is leading to Labour's next big defeat.

However, we cannot yet be certain. Political leaders, as Hillary Clinton has often reminded us, have to “close the deal”. She has not, in fact, been able to close her own deal.

The Conservatives in Britain are now way ahead in the polls, but they have not won a by-election for a generation. They need to show that people's voting intentions will actually lead to pencil crosses in polling booths. I expect they will at Crewe, but in politics there is nothing like the fact of victory.

That is why Boris Johnson's election as Mayor of London was so significant.

The Labour Party in Parliament is beginning to be demoralised by this swing of the electoral pendulum. Even when things are going well, MPs tend to be paranoid about their seats. One hears MPs with majorities of 15,000 speaking of their seats as highly marginal. Yet now there are at least 200 Labour MPs whose seats really are at risk. In terms of the majority, Crewe is Labour' 147th most-vulnerable seat. There are 146 Labour MPs whose seats are more marginal than Crewe.

So far, the issue of changing Labour's leader has been hypothetical. It is very difficult to work the Labour constitution so as to depose a leader; Gordon Brown is a tough figure for any challenger to take on. If Crewe is lost, gossip will become more serious. If a challenger emerges who can plausibly offer to help to save their seats, the gossip among Labour MPs will become a real threat to Mr Brown.

Because the electoral cycle is swinging in their favour, the Conservatives have the appeal of the rising sun. David Cameron still has to face erratic small arms fire from some newspaper critics, but the public has moved towards him. Mr Brown will always remain a formidable politician; the “big clunking fist” may look rusty, but he will never be negligible. Mr Cameron may have little to fear from Nick Clegg, the Liberal Democrat leader, who has not captured the public imagination. Mr Cameron is himself much more convincing and a natural leader of the liberal centre.

The Conservatives should not now make too many commitments of policy, though they will be urged to do so. The public recognises the threats to economic prosperity. The cycle of boom and slump, which Gordon Brown rashly promised to abolish, has again turned down.

David Cameron and George Osborne cannot know what financial difficulties are ahead. There is everything to be said for them avoiding commitments that could prove to be inappropriate in the actual conditions a Conservative government might face. Common sense and caution are the right qualities for playing a winning hand.

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