Joining the dots from 'back stabbing' to Trident

Try to make the connection:

Michael Fallon, Defence Secretary: "Ed Miliband stabbed his own brother in the back to become Labour leader. Now he is willing to stab the United Kingdom in the back to become Prime Minister."

Nicola Sturgeon, SNP leader: "It is often asked of me, is Trident a red line? Well here is my answer: you'd better believe Trident is a red line!"

What has the fraternal feud between Ed and David Miliband got to do with the UK's at sea nuclear deterrent?

Welcome to Thursday on the campaign trail.

After two days in which the Tories have found themselves responding to Labour's agenda (Tony Blair, non-dom tax),the Defence Secretary has shuffled his pack and dealt the Trident card.

So here's the answer to the question posed above: Ed Miliband's willingness to fight his brother for the Labour leadership suggests he would be willing to do a deal with the SNP on Trident in order to get the keys to Number 10.

Clear?

Many people have said this morning that it's a pretty desperate argument - albeit they are mostly - but not all - Labour supporters.

Labour, meanwhile, has reaffirmed its commitment to the costly renewal of Trident (although it's unclear if they plan to do it with three or four submarines).

But the Conservatives are not recoiling one inch.

The SNP are implacably opposed to the renewal of Trident, so Michael Fallon is suggesting the UK's national security would be put at risk if Ed Miliband walks into Downing Street with the support of the Scottish Nationalists.

One big flaw in this argument: we've just had a coalition government for five years in which one party supported Trident and the other didn't.

If that difference of opinion on defence didn't get in the way of those two parties governing the country - why should it be any different in what would be a much looser power sharing arrangement, between Labour and the SNP?