The Conservatives are now, at last, a credible opposition — just as they were in the late Sixties and Seventies before they went on to win the subsequent general elections. Far more than the precise numbers of yesterday’s by-election, it is the Tories’ ability to end the 26-year-old jinx of their failure to win a seat from another party, and the 30 years since they last won from Labour, that has re-established them as serious contenders.

The past 11 years have been an oddity, a period when the Conservative Opposition failed to take advantage of the troubles of the Labour Government — as the Liberal Democrats have — to win a seat from the governing party. But now they have. So David Cameron can claim, as he doubtless will when he visits Crewe & Nantwich today, that the Tories are now on the way back to power.

That is why the by-election is far more significant for the Conservatives than it is for Labour. Of course, losing a seat is bad news for Gordon Brown after the unremitting onslaught he has faced during the past few weeks. But, despite the fevered reports of the past few days, this defeat is not going to mark the end, or even the beginning of the end, for Mr Brown. Remember, it is only just over two years ago that the Liberal Democrats captured Dunfermline & West Fife from Labour, the constituency next door to Mr Brown’s own, on a 16.4 per cent swing. The Crewe & Nantwich result will fuel excitable headlines but it is no worse than governing parties have suffered in the past.

In many ways, what we saw yesterday was a return to normal: a governing party losing out to the main opposition party — just as happened when Labour was last in power in the Sixties and Seventies. From 1966 to 1970, the Tories gained as many as twelve seats from Labour, and other parties captured two. And, from October 1974 until 1979, the Tories gained six and the Liberals one. So the Tory victory in Crewe & Nantwich should not be exaggerated.

Moreover, while the scale of the victory will dominate the headlines for the next day or two, the precise numbers are a poor guide to what will happen at the next general election, or even at the next parliamentary by-election.

In the 1966-70 parliament, the swings varied enormously, from 8.5 to 21.2 per cent. Even on the same day, in March 1968, when the Tories won three seats from Labour (Acton, Dudley and Meriden), the swings varied between 15.1 and 21.2 per cent. A decade later there were also wide variations in by-election results: in April 1977 Ashfield was lost by Labour on a swing of almost 21 per cent on the same day as Austin Mitchell held Grimsby for Labour on a mere 7 per cent swing. And the last Labour by-election loss to the Tories, in March 1978 in Ilford North, was on a swing of only 6.8 per cent.

So the victory matters more than the precise numbers. It is futile to extrapolate to the general election on the basis of last night’s figures. They will not be repeated. What matters is the mood — and the Tories have established themselves as credible challengers to Labour. Mr Cameron has shown that his party can win.

Times Recommends

• Fundamentalist brew: faith and power
• Big guns wheeled out for the marginals
• Wanted: new merchants of light