TORY strategists must be biting the carpet in fury.

For after the comeback, there's the setback - of yet another MP defecting.

How frustrating it must be for the party's campaign planners.

Such was John Major's shirt-sleeved success at the Conservative conference - painting the Tories as the champions of the working class and Labour as middle class careerists - that the notion of a snap general election was not being ruled out.

Now, the disenchanted Tory MP for Bolton North East, Peter Thurnham, crosses the floor to the Liberal Democrats.

It is the third defection suffered by the Conservatives in 12 months and all the good work of the past seven days is undone.

Not least because Mr Thurnham brings the issue of Tory sleaze back to the boil by citing the Prime Minister's "lack of willingness" to tackle the allegations as one of the reasons for his departure.

Mr Thurnham's high-mindedness is, of course, easily shot down.

For few MPs doubt that the real reason for his defection was his failure to secure a safe seat - or even an interview for the secure one at Westmorland and Lonsdale, the constituency in which he lives - after announcing he was standing down at Bolton where he has only a 185-vote majority.

But, though he has sulked since January when he resigned the party whip, Mr Thurnham has stuck the knife in now with vicious timing. It is designed to take the gloss of the Tories' conference triumph of forging unity across the divide over Europe and allowing the sleaze row which clouded its start to subside.

Liberal Democrat leader Paddy Ashdown, obviously a party to the defection plot, must have been laughing up his own shirt sleeve.

As John Major soaked up the praise at Bournemouth he prepared to blow it all away and welcome his new recruit, who boosts his party's numbers in the Commons to a post-war record of 26.

The defection is, of course, a temporary setback for the Tories.

Mr Thurnham had, in effect, already left the Tory fold and Paddy Ashdown cannot for long parade his latest capture to lasting effect.

The real damage of this departure is the upset to the Tories' comeback timetable.

After a successful week at Bournemouth and with the effects of an improving economy - the most crucial factor in the way people vote - soaking into the electorate's perception, the party's planners have to begin all over again to cultivate the positive climate, a process which can take weeks and months.

And when starting from way behind, with the sands of time running out before an election must be called, the party needs a setback like this like a hole in the head.

Peter Thurnham may be a long-forgotten figure by the time the poll comes, but he could be a crucial little nail in the coffin.

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