David Cameron's success in decontaminating the Tory brand meant disillusioned Labour voters saw his party as an acceptable vehicle by which to deliver a bloody nose to Gordon Brown's government.

The Conservatives are electable. They are favourites to win the next general election too. But it is not a foregone conclusion that they will win it. Instead, this month has marked the end of a decade and a half in which the normal laws of political gravity were suspended. There was only one electable party in British politics between the ERM crisis of Autumn 1992 until David Cameron's election as Opposition leader at the end of 2005. That period is over. Labour has not yet understood or adapted to this change, relying too heavily on an outdated 1990s campaign textbook and script which will no longer work.

Crewe and Nantwich counts as a famous victory for the Conservatives in part because of the scale of their victory but also because of the unparalleled historic failure which has preceded it. The 'toxic Tories' made no by-election gains during the last eleven years in opposition, though governing parties had lost seats directly to the official Opposition in by-elections during every Parliament since 1955 (excepting only the short six month parliament in 1974).

Though many political memories at Westminster do not go back before 1997, the Crewe swing registers only in the middle of the richter scale of historic by-election earthquakes.

Both major parties have suffered several larger swings to third parties, as happened to Labour in Brent East and Leicester South during the last parliament. But, taking Labour-Tory battles, the Crewe swing was smaller than those of 29% in Dudley West, 22% in South-East Staffordshire in the years before Tony Blair's 1997 landslide.

Nor was it as big as the 21% swing in Mid-Staffordshire to Kinnock's Labour Party in March 1990, though it does considerably outstrip than the 10.5% swing won by Neil Kinnock's Labour in Fulham in 1986. While the Tories won swings around 21% in Ashfield in 1977 and in Dudley in 1968, it took the Winter of Discontent to confirm the 'sea change' of 1979 while Labour bounced back into the lead after its 1968 local election hammering, before a surprise defeat in a neck-and-neck election.

So Labour is in dangerous territory, but the history suggests this is not mission impossible yet. Everything depends on whether Labour can use the next two years effectively to offer a real choice at the next election.

Firstly, Labour needs to understand that 'Keep the Tories Out' will not be enough to win. As late as 2005, the argument that 'the Tories haven't changed' was just about enough to see an unpopular government home. Of course, Labour must challenge and scrutinise the Tory claim to be ready to govern but that won't succeed without its own positive message too. That argument wasn't clear enough in London or in Crewe. And Labour must play the ball, not the man: what their opponents believe, not where they come from.

Secondly, the Crewe defeat clearly shows that Labour's heartland support is unhappy, unclear about what Labour stands for, and unsure that the party wants to stand up for them. Every serious contribution to the post-local election inquest has emphasised that a Labour fighback must reunite a broad electoral coalition, addressing northern and southern discomfort, not debating whether it should choose to appeal to heartland or swing voters.

This also means that the uber-Blairite critique of Brown is less convincing after Crewe. What, apart from presentational advice, is being offered on substance that would have improved this result, rather than making it worse? The voters of Crewe were not crying out for bolder reforms of the public services. They wanted to give Labour a bloody nose over the 10p tax rate and the closure of post offices.

Thirdly, Labour must now think and fight politically as an opposition party would, while using its power to govern over the next two years to offer concrete challenges to its opponents. The aim must be to restore its own reputation for fairness, and test the opportunistic 'magpie' raids which the Conservatives have made.

A full version of this article is first published on Comment is Free . 

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