If Labour loses Glasgow East, a seat it claims as part of its Scottish birthright, it will confirm that Labour's UK hegemony is in its death throes less than two years from a general election and confirm that the job Gordon Brown schemed and plotted and fought for for 20 years has become the most poisoned of chalices. If he loses on July 24, his MPs will ask themselves who they would rather save - him or themselves. The answer for Brown, coming after the disasters in Crewe and Henley, will be unpalatable.

If Labour scrapes home, Brown will get no credit and the caravan will move on to the next staging post. Lose and the roof falls in. The myth of Scotland as Brown's fiefdom will be put to rest and the shelf life of Prime Minister Brown will be numbered in months at best.

Consider the facts. Voting Labour has been part of the DNA of the East End of Glasgow for decades. Some would say that voters there have received little thanks for it, with many living shabby lives in a vortex of deprivation, violence, unemployment, poverty and drug abuse. And yet it remains the 25th safest constituency out of all 646 in Britain. Labour there has never felt remotely threatened - until now.

The threat will come from the SNP, afloat on a sea of populism in the Scottish Parliament. It offers an easy promise, that independence, fuelled by the North Sea oil price bubble, will be an instant fix for places such as the East End.

The Nationalists, with their propaganda machine working at full capacity, will paint Brown's Britain as a land flowing with economic bad times and unfulfilled promises. The East End, they will say, is the kind of place that new Labour, in its pursuit of Middle Britain, cynically put to one side.

That the Nationalists, 13,507 votes behind Labour in 2005, are now in any kind of position to think of victory is the best possible indication that Labour's time in government at Westminster is almost up. In Scotland, that has already happened. The SNP's ousting of Labour at Holyrood last year showed that the inevitable no longer happened in Scottish politics.

Labour in Scotland is struggling to make ends meet; its one-time army of local activists has deserted, shy of being identified with a party on the decline. Its Scottish leader has just quit.

Labour will say: “This is a Westminster by-election and only Labour or the Conservatives can be in Government.” It is a hazardous approach. Perhaps, though, it is Labour's only way of continuing to believe that, in the East End of Glasgow, the worst is not about to happen.

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