This is no isolated shock: it portrays a deep, dark electoral hole for Labour

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BY-ELECTIONS can often flatter only to deceive. In February 1983 in Bermondsey, the SDP pulled off what remains the biggest by-election swing ever. Yet the party still came third in the general election just three months later.

Equally, the Conservatives lost both Kincardine and Deeside to the Liberal Democrats and Langbaurgh in the north-east of England to Labour on the same day in November 1991. Yet the following year, John Major won a fourth Tory term.

By-elections can mislead for two reasons. Sometimes local circumstances, issues, or individual candidates have an unusually strong impact. Meanwhile, the anti-government swing is often much bigger than anything that would transpire in a general election. Voters use the occasion to send a protest note to the government without fear that the opposition could secure the reins of power.

So it is highly unlikely there would be a Glasgow East-style 22 per cent swing to the SNP in any imminent general election in Scotland. Calculations showing such a result – leaving Tom Clarke in Coatbridge as Labour's only Scottish MP, and giving the SNP 50 seats – are little more than a "bit of fun".

But that does not mean we can dismiss what happened in Glasgow East. There is, after all, nothing unusual about the constituency or the campaign to have generated such an unusually high swing. Indeed, if anything, the opposite is true, given it is such a socially deprived and strongly Catholic constituency.

Moreover, Glasgow East is no isolated reversal. At 19 points, the fall in the party's vote matched almost exactly the 18-point drop in Crewe and Nantwich in May. It comes, too, in the wake of UK polls that put the party on average at just 26 points, 19 points behind the Conservatives, as well as the first-ever Scottish poll to put the SNP ahead of Labour in Westminster vote intentions.

So Glasgow East is no isolated shock. Rather it is but yet another piece in a jigsaw that portrays a deep, dark electoral hole into which Labour has fallen. While a 22-point swing to the SNP in a general election may be unrealistic, the claim of a recent poll that there might be a 13-point swing certainly looks plausible.

And even a swing of that size would be sufficient to produce a significant Nationalist breakthrough of 15 seats, more than the party has ever had before at Westminster. With 29 seats, Labour might still have the largest body of MPs, but no longer would it have a near hegemony of Scottish representation.

Meanwhile, governments that regularly suffer big double-digit swings against them in by-elections – as happened to the Labour administrations of 1966-70 and 1974-9 and the Conservative government of 1992-7, have always gone down to defeat in the subsequent general election.

Little wonder then that not only can by-elections provide a pointer to the future but that also, on occasion, they help change the political landscape.

For example, the SNP's victory in Govan in 1988 helped to ensure Labour played a key role in Scottish Constitutional Convention – a vital step on the road that led to devolution.

Equally, the Conservatives' unexpected defeat at the hands of the Liberal Democrats in Eastbourne in 1990 helped set off the train of events that eventually lead to Mrs Thatcher's downfall.

That, of course, is an uncomfortable precedent for Mr Brown in the wake of Glasgow East. It is a precedent he must hope will not now be repeated.

• John Curtice is Professor of Politics at Strathclyde University