Disproportionate 2010 LD>LAB switching in the marginals could shift more seats

We all know that the national vote threshold for LAB overall majority is considerably lower than for the Tories. The reason is partly the boundaries but mostly down to the way the Labour vote is distributed.

Generally LAB supporters appear much less likely to turnout in seats where the outcome is a foregone conclusion.

But could the overall bias against the Conservatives be even greater at GE2015 than at previous elections?

For one pattern picked up by last month’s massive 12,800 sample phone poll of 40 CON held marginals by Lord Ashcroft was that 2010 LDs appeared to be more likely to switch to LAB in the battlegrounds than in a national comparison poll carried out at the same time.

The chart above shows the difference.

Overall Ashcroft found a 6% CON>LAB swing compared with an 8.5% one in the marginals a differential partly driven by the LD switchers. This was not a fluke finding from one poll. In an earlier Ashcroft marginals poll in August 2011 LAB was doing better in the marginals than elsewhere.

Indeed one of the factors that adds credence to this polling is that the pattern of the latest polling was similar to the earlier the main difference now being the impact of UKIP.

The main things you look for in polls of the marginals are whether there’s a different pattern of support – and both Lord Ashcroft’s polls have found that.

But there’s little doubt from both sets of surveys that a big driver of the difference was the group of 2010 LDs.

Given that it’s reckoned that on a normal swing LAB could achieve a majority with a lead of 2-3% then it is theoreticaly possible that EdM’s party could poll fewer votes overall than the Conservatives and still squeeze a majority.

Mike Smithson