All change at Crewe for the Conservatives?

After their landslide success on May 1 in London and across the country, the Conservatives look certain to win the parliamentary by-election on May 22 in Crewe and Nantwich, called after the death of the sitting Labour MP Gwyneth Dunwoody. With only an eight per cent swing needed to gain the seat, it's surely a cinch for David Cameron's newly energised troops. Or is it?

There are three reasons why the Tories cannot afford to be complacent. First, the modern party has yet to prove it can do by-elections. They haven't managed a by-election gain since 1982.

Second, while Labour will not be expecting to hold on to the seat, they have given their opponents very little time to prepare - calling the election even before Ms Dunwoody's funeral this Thursday - and they have chosen a unique candidate in Tamsin Dunwoody, Gwyneth's daughter.

Third, the Lib-Dems do do by-elections, and

no one does them better than their portly chief executive Lord Chris Rennard, architect of many a by-election triumph over the past 30 years.

The key to winning a by-election is for the second-placed party to squeeze the third-party vote and Rennard is the master of this. But it is the Lib Dems who ran third last time with less than 20 per cent of the vote. So Rennard's job is to stop a Tory squeeze on Lib Dem electors. And the Tories are well aware that their failure to operate a squeeze in last year's Ealing Southall by-election handed Labour a comfortable victory.

Stopping a Tory victory is important to the Lib Dems' leader Nick Clegg. Any doubts about that were dispelled by the ruthless way Rennard dumped Marc Godwin, the 49-year-old candidate who had been selected to fight Crewe in the next general election. Godwin quit the party in protest when he was replaced by a younger 'photogenic' woman, Elizabeth Shenton.

If Rennard and Shenton do their stuff, they could wipe the smile off Cameron's face and hand Brown a prize he hardly dares expect.