Sarah Brown’s blue eyes widened with delight as starstruck Labour activists in Glenrothes presented her with a birthday cake.

For all but one of the past 10 days, the prime minister’s wife has been braving the wintry elements in the Fife constituency to help save her husband’s political career. The former public-relations executive hopes she will have more than her 45th birthday to celebrate by the time the by-election votes are counted on Thursday.

Travelling from the family home in North Queensferry, she has knocked on doors, staffed phone lines and charmed voters. In the parlance of Sir Alex Ferguson, the Manchester United manager who endorsed Labour’s campaign last week, you need your best players in a crunch match. For the prime minister, that means his wife.

It is the first time a prime minister’s spouse has campaigned in a by-election, and her deployment is a sign of how significant the poll is for Labour and how desperate Brown is to win.

A victory over the SNP would be seen as confirmation of his boomerang trajectory from Stalin to Mr Bean to Superman. A loss in his back yard would raise more doubts about his chances in a general election.

The Labour leader’s non-appearance in Glasgow East was blamed for the party’s narrow defeat at the hands of the SNP in July. He has discarded political convention to lead this by-election from the front, with two campaign visits in the past fortnight.

But Labour’s own canvass returns indicate the fightback is too little, too late, with Lindsay Roy, the granite-faced head of Kirkcaldy High School, the prime minister’s alma mater, expected to fall up to 3,000 votes short of the nationalists.

So will November 6 record the hammering of another nail in Brown’s coffin and another milestone on the nationalists’ march towards independence. Or can Labour save the seat and the Union?

THURSDAY should be Labour’s day. Glenrothes, a new town fashioned from the economic boom of the 1960s to provide new housing for Edinburgh’s displaced working classes, adjoins former mining and industrial areas. It is traditional, rock-solid Labour territory so it is no surprise that Sarah Brown has been given such a warm reception.

In Cardenden, on the outskirts of the constituency, voters were delighted she had taken the trouble to visit. Janet Anderson, 65, told her she looked “absolutely gorgeous.” Debbie Beattie, 38, who works in Glen Baker’s, was similarly smitten.

“She looked a lot younger than she does on television and, yes, I will be voting for Labour,” she said.

Willie Mackie, 68, a retired fork-lift driver, met Mrs Brown on her recent visit to the Glenrothes Recreation and Social Club and that was enough to convince him to stick with Labour. “She listens, and she encourages people to get out and vote.”

Times Recommends

• Should Knox’s trial even have reached the courtroom?
• All that glitters is not sold in Wright’s world
• Believe it or not, Big Brother is your friend