Who's Telling Porky Pies?

"If people end up opting out by drifting off to the Lib Dems then it only takes one in ten of our voters to drift off to the Lib Dems you end up with a Tory Government."
Tony Blair, 30 April 2005, C4

This is commonly referred to as the "back door" argument. It goes like this:
"It is Labour versus Tory. Anything else is a Tory vote by the back door and they (the voters) should have nothing to do with it."
Tony Blair, 29 April 2005, BBC

A warning - swearing may occur in the following.
In the world of intelligent bloggers it is an argument commonly referred to as bullshit.
Nick Barlow knocks it down here, and again here.
Phil at Cabalamat Journal does a good job on it here.
Justin at Chicken Yoghurt has been keeping a "back door" count here, here, here, here, and here.

And just in case you're thinking this is some sort of interweb blogger conspiracy to discredit the "back door" argument, you can also read about it in the proper media.
The Independent does a good, if somewhat belated job with this:
Vote for Lib Dems will not let in the Tories
"Labour's attempts to warn its wavering supporters that a vote for the Liberal Democrats could allow Michael Howard into No. 10 "by the back door" was undermined yesterday in a detailed study carried out for The Independent. The study found that a swing of 11.5 per cent from Labour voters to the Liberal Democrats could deprive Mr Blair of his overall Commons majority but it would be virtually impossible for such defections - at even twice that rate - to let in the Conservatives to form a government.
John Curtice, the respected psephologist and professor of politics at Strathclyde University, who carried out the analysis, said: "Labour's claim that switching from Labour to the Liberal Democrats could enable Mr Howard to win the election is highly misleading."

And Channel 4 Factcheck are similarly unconvinced.
Paul Whiteley, professor of politics at Essex University who is advising the British Election Study, also rejected Labour's claim.
"It's not true. Currently our surveys in the British Election Study put Labour at 36 per cent, the Conservatives at 31 per cent and the Lib Dems at 23 per cent. If 10 per cent of the Labour vote were to switch to the Lib Dems it would go to roughly 33 per cent Labour, 31 per cent Conservative and 26 per cent Lib Dems.
In this case Labour would still get a comfortable working majority."

I think that's fairly conclusive. The "back door" argument is a porky pie told by the Prime Minister in order to scare people into voting Labour. Who'd have thought the man could stoop so low?