At a packed Edinburgh Book Festival event last week the Scottish author Ian Rankin spoke of the political changes that have swept this country in the last 18 months. The next three to five years will be an exciting time to be in Scotland, he said, with glee.

Well, perhaps if you are a phenomenally successful crime writer they are, but for a beleaguered Scottish prime minister, “exciting” must sound like a euphemism for something much deadlier.

When Rankin interviewed Gordon Brown earlier at the same festival the mood was light, but Scotland has come to represent a dark chapter in Brown’s premiership.

Now his home turf has presented him with the second nightmare scenario of the summer in the shape of a looming Westminster by-election.

Before Labour has had a chance to recover from its disastrous defeat in Glasgow East in July it must gird itself for another drubbing, after the death of John MacDougall, MP for Glenrothes, on Wednesday.

If the SNP could overturn a 13,500 majority in Labour’s Glasgow heartland, anything is possible — even conquering that bit of Fife that nudges Brown’s neighbourhood. A loss in the west could be considered a Labour loss, but failure in Fife would be all Brown’s and it’s hard to see how he could survive it.

The odds are stacked against him. Labour’s hegemony is long gone here, as it is throughout Scotland. Where the party once held four out of the five Westminster seats in Fife, it now has just Glenrothes and Brown’s Kirkcaldy and Cowdenbeath the LibDems sensationally won Dunfermline and West Fife from Labour, and the SNP took the Holyrood seat of Central Fife in 2007. The party doesn’t even get much of a look in at local authority level, with the council under the control of a coalition of nationalists and LibDems since May last year.

As Alex Salmond and his disciplined troops appear to carry Scotland with them on a tide of approval, Labour has never looked more downtrodden. It is a reflection of the party’s and the prime minister’s unpopularity, and the unstoppable march of the nationalists, that Glenrothes is being talked of already — even before MacDougall is buried — as Brown’s denouement.

If ever a politician didn’t need a test of public opinion, it is Brown now. The timing could not be worse. He of all people, however, must have seen this coming. Brown visited MacDougall in hospital on Monday night and has been aware for months, if not longer, of his friend’s frailty.

The prospect of another Scottish by-election must have been uppermost in his mind during the summer, even as he prepared to re-energise his government with a possible cabinet reshuffle, barnstorming conference, and pre-Budget giveaways in the autumn. What’s more, he has been in politics long enough to know that he goes into the Glenrothes contest with an advantage of sorts: with his party demoted to underdog, the heat is off.

The bookies and almost everyone else have made the SNP the early favourites and thus shifted the pressure on to Salmond, who will look like the loser if his party does not wipe out Labour’s 10,600 majority. Salmond will relish such a challenge, but how will Brown play his hand?

He is expected to delay calling the by-election until October, which after the haste in Glasgow East is cynical but probably tactically judicious. This gives him time to regroup and re-brand before throwing himself again at the mercy of voters.