The Tories’ stunning victory in Crewe & Nantwich last week had commentators searching the record books. Many of the numbers recalled the years before Margaret Thatcher swept to power in 1979.

It was the first Tory by-election gain from a Labour government for more than 30 years; the biggest swing from Labour to the Tories since the Ashfield by-election in 1977; the first time the Tory share of the vote at a by-election has increased by more than 10% since Knutsford in the dog days of the Callaghan administration in March 1979; and the first Tory gain in the north since Workington in 1976.

The contrast with the party’s recent history is striking. In most by-elections since Tony Blair became prime minister in 1997, the Tories have struggled to protect their share of the vote or, worse, have lost out to the Liberal Democrats.

But it is some of the more subtle statistics of the Crewe result that are of longer- term significance. First, turnout: instead of slumping as is usual in by-elections, almost as many people voted on Thursday as had done so at the 2005 general election.

Concern about the economy, disenchantment with Gordon Brown and the government and anger about the abolition of the 10p tax rate all combined to mobilise electors. Not since voters vented their anger over the poll tax in Mid-Staffordshire in 1990 has turnout held up so strongly. Such a level of turnout removes the defeated party’s tried and tested excuse that it lost because its supporters stayed at home.

An interesting measure of each party’s performance is its so-called “retained vote” – that is, its by-election vote as a percentage of the vote it won at the previous general election. A low retained vote means a party’s supporters are either abstaining or, worse still, switching to a competitor. A high retained vote indicates stable or even growing support.

At by-elections parties often end up polling fewer votes than at the previous general election, whether they win or not. In Crewe & Nantwich, however, an old-fashioned two-party dogfight saw many former Labour voters switch directly to the Tories. There can be no other interpretation of the result.

More than 8,000 fewer people voted Labour last week with the party retaining less than 60% of the support it had in 2005. The Lib Dem vote was also down, though they did rather better, holding on to three-quarters of their 2005 vote.

The Tories, on the other hand, attracted more than 6,000 additional by-election voters to poll an impressive 145% of their general election total.

These new supporters must now be wooed for the longer term. Crewe & Nantwich will receive much less attention in the midst of the next general election campaign, but Edward Timpson must hang on if his leader is to enjoy a working majority in the House of Commons. Otherwise, this by-election upset will join a long line of others that suggested an electoral sea change only to revert to the status quo ante when the spotlight moved away.

It is perhaps worth noting that Neil Kinnock’s Labour party gained four seats from the Tories in the 1987-92 parliament, only to lose them all at the subsequent general election.

However, if by-elections have not always presaged a change of government, they do sometimes spark pressure for a change of leader. Mid-Staffordshire and, later in 1990, the loss of Eastbourne to the Lib Dems, were key events in encouraging Michael Heseltine, the former Tory cabinet minister, to challenge Thatcher for the party leadership.

John Redwood challenged John Major for the Tory leadership following a run of by-elections in 1994 and 1995 when the party’s vote share fell by an average of 20 percentage points.

Labour has been less brutal in deposing its leaders, though spectacular by-election reverses such as those in Dudley and Meriden in 1968 sparked talk of a cabinet coup against Harold Wilson.

Nevertheless, this spring’s elections – the local polls, this by-election and the one for the London mayor – may well have changed the terms of trade in British politics. It seems that it is no longer unfashionable to be a Conservative.

The era when Tories would not admit their preference to pollsters, and would not display a poster in their window appears to be over.

It is always fanciful to translate swings at by-elections into general election results. No one believes the Tories would achieve the near 18% swing from Labour seen at Crewe & Nantwich, which would leave Labour with little more than 100 MPs in the Commons.

The Tories’ general election task remains a stiff one. They need a swing of 7% to win an overall majority, and only Tony Blair has done better than that in the modern era. He achieved a swing of 10.2% in 1997. The Tories’ best was Thatcher’s 5.3% swing in 1979.

The Conservatives still have a lot to do if they are to win outright. Their policies and leadership will now come under greater scrutiny for that very reason. But if, as it seems, the electoral winds have shifted decisively this spring, for the first time in a decade a Tory-led government looks a real possibility.

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