How the fight is on a knife edge
Given the political and betting importance of which party wins most seats I’ve been looking at the maths to try to get a broad figure of what would be a CON victory on seats or a CON defeat. I’ve made several assumptions that clearly will impact on the equation.
In broad terms the gap between the main parties as we start the formal campaign is 46 seats. So if there was no Scottish problem Labour would need to make 23 gains from the Tories to get level. This is because each CON loss to LAB adds one to the red team’s total and reduces the the blue total by one.
Assumption 1: Both LAB & CON do equally well in picking up LD seats. This is supported by the Ashcroft constituency polling. In the actual election things could change but this is the best rule of thumb at the moment.
Assumption 2: LAB to lose three-quarters if its contingent of Scottish MPs to the SNP. There are currently 41. Assuming that the LAB-SNP swing is in the lower range of current polling this would mean 30/31 seats going.
Assumption 3: LAB to lose two fewer seats to other parties than CON. This is mainly the UKIP factor which is likely to hurt both major parties but more purple targets are currently CON held than LAB.
Assumption 4 The speaker to be re-elected in Buckingham and by the new parliament. If Bercow is back as an MP but not re-elected Speaker he is unlikely to support his former party.
Taking all this into account my best guess is that if CON can keep its net losses to LAB down to fewer than 38 seats it would be the biggest party in the new House of Commons. In vote share terms on a uniform swing this amounts to a CON-LAB swing of about 3%.