Is the by election seat the best guide to the election?

Simply to get a majority, based on the new constituencies boundaries it’s estimated that the Tories need to gain 114 seats. Check out the list on UKPollingReport here.

In strict numerical order these include 29 seats where the incumbent or notional incumbent is not Labour and where conventional swing analysis might not apply on quite the same scale. They include Tory targets in Scotland, including some held by the SNP, as well LD>CON marginals throughout Britain.

So if not all the exceptions fall then we have to look further up the list to Labour seats where a bigger swing is required in order to make up any possible short-fall.

To work out the national swing from a poll or other projection add the increase in the Tory percentage since the general election to the decrease in the percentage of the party you are comparing with and then divide by two. So yesterday’s ICM poll share of 42% reflects an 8.8% increase since May 2005. The 30% Labour poll share suggests a 6.2% drop on the election British share of 36.2%. So adding to the two changes together and dividing by two we get a swing of 7.5%

On the UKPR list such a swing takes us to Tory target 143 – Batley and Spen – and even with some seepage of seats in Scotland and where the LDs are defending the Conservatives are still well above the 325 seats that are required for certain victory.

This brings us to Crewe & Nantwich – scene of the spectacular by-election victory last May and just about the only such election in the past four years which is relevant to the looming battle. All the other by elections have been “odd” in some way and have not involved a CON-LAB scrap in a Tory target.

Holding onto C&N, Tory target 165, requires an 8.4% swing on 2005 – a touch ahead of current national poll ratings – and would suggest, if it happened, a very comfortable Cameron majority perhaps in the 75 – 100 region.

Looking over the by election figures again what impresses me is that turnout was almost at general election levels and that in real votes the Tories went up from 14,162 at the general election to 20,539 in the by election. The election itself took in a highly charged political atmosphere – Boris had won London only three weeks before – and, unusually, there was a high level of national media coverage.

My guess is that on general election day the contest will be much tighter but it’s hard to call whether it will stay blue or not. It could be within a few hundred votes.

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