Sarah Brown has emerged as Labour's chief campaigner in the Glenrothes by-election as polls suggest that the party has pulled level with the Scottish National Party with three days of campaigning to go.

Labour's revival in the constituency, against all predictions at the outset of the campaign, has been credited in part to the activities of Mrs Brown. She has been campaigning without publicity on five or six occasions, and was yesterday again pounding the streets on behalf of the Labour candidate, Lindsay Roy, a local headmaster.

But with canvassers reporting that many voters still refuse to commit themselves, no one, with the exception of the Nationalist leader, Alex Salmond, is prepared to forecast the result. Mr Salmond, who has been visiting the seat almost daily, said simply: “We will win.” The bookies are more cautious. From a position where they were taking bets at five to one against Labour, they have shortened the odds to evens on both parties.

It is a mark of the Government's precarious position that a seat in which Labour holds a 10,664 majority has been regarded from the outset as a shoo-in for the SNP.

Only three months ago, the Nationalists triumphantly overturned a majority of more than 13,000, in what was regarded as the heartland Labour seat of Glasgow East - a swing of 22 per cent. With the death of the sitting MP for Glenrothes, John McDougall, Glenrothes loomed as yet another by-election disaster for a hugely unpopular Government.

The swing needed by the Nationalists this time is “only” 14 per cent, compared with the 22 per cent they achieved when bringing off the political earthquake in Glasgow East.

But that was then, this is now. The revival of Gordon Brown's fortunes - a successful conference, the leadership qualities he has shown during the financial crisis, and the rescue of the two Scottish banks - has swung opinion back towards Labour. The party's canvassers on the ground remain cautious, with most saying that there may not be enough days left for Labour to close the gap and overtake the SNP, but momentum is certainly on its side. One of the many paradoxes of this fascinating contest is that the party that should, on paper, be the challenger - the SNP - finds itself on the defensive.

Last Friday Mr Brown confronted another paradox - this may be a Westminster election, but not everyone sees it that way. At a carefully staged discussion with voters, he found himself sitting with a man who should, by all rights, have been a natural Labour supporter. Mitchell Collins is a “locum” Church of Scotland minister - standing in when the incumbent is not available - and he is a next-door neighbour of Mr Roy.

But he turned out to be an SNP supporter, and although Mr Brown tried for ten minutes to win him round, Mr Mitchell was not for shifting.

His argument was simple: Mr Brown was, he thought, the best leader for Britain, but Mr Salmond was the best leader for Scotland.

Whether that view will prevail on Thursday remains to be seen.

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