Statistically, this Labour Government has suffered worse by-election reverses during the past 10 years. But, politically, its loss of Glasgow East is the most devastating.
At 22.5 per cent, the swing from Labour to the SNP was a little less than the 23.2 per cent swing in Hamilton South in 1999 when George Robertson resigned his seat to become Nato's secretary general. It was certainly well below the 29 per cent swing from Labour to the Liberal Democrats when Sarah Teather captured Brent East in 2002.
But such previous record-breaking defeats could be blamed on local factors. Hamilton had been the location of the SNP's breakthrough by-election success in 1967. In Brent East there was a substantial Muslim population deeply concerned about Britain's involvement in the Iraq war.
There were no such excuses in Glasgow East. The heavily socially deprived area has returned Labour MPs for nearly a century. One in three voters is Catholic, a group still likely to favour the party. The Labour candidate, Margaret Curran, is already an MSP for much of the constituency.
Yet still Labour lost – as it did a few weeks ago at the Crewe and Nantwich by-election. Moreover, at 19 points, the drop in Labour's vote was almost exactly the same as the 18-point fall in Crewe. Together with the party's poor performance in the local elections in May, a consistent pattern has developed. Serious Labour losses at the ballot box are no longer the exception but the rule.
John Curtice is professor of politics at Strathclyde University