New Ipsos MORI/Reuters poll of marginals

Ipsos-MORI have published a second wave of their marginals polling for Reuters (for my report on wave 1 see here), which shows a swing of 5.5% from Labour to the Conservatives, up from 5% in the first wave in mid-March.

This size swing is the equivalent of a lead of 8 points nationally. MORI's last GB poll showed a lead of 5 points, so in theory this would suggest a bigger swing in the marginals though one should add the caveat that MORI's last national poll was 2 weeks ago now, so the national picture will have changed somewhat.

To get an overall majority the Conservatives need a swing of 6.9% in marginal seats, so this shows them falling short. The topline figures for voting intentions in these seats are CON 38%(+1), LAB 41%(nc), LDEM 11%(nc). To put that into context, for this particular group of seats (I assume by design rather than co-incidence) the required 6.9% swing equates to the two parties being exactly neck-and-neck. So in theory, if the Conservatives are ahead in MORI's marginals surveys, they should have an overall majority. In practice of course that assumes Lib-Dem marginals behave the same as these ones, which is somewhat dubious assumption, so in reality the Conservatives could need a bigger or smaller swing against Labour, depending on how they do against the Lib Dems.

In previous waves of marginal polling I've seen some slightly confused interpretations of them - it's important to remember that this poll was conducted only in Labour held seats - and in the case of the marginals MORI are looking at, Labour seats where the Conservatives need to overturn sizeable majorities. All the questions in the poll need to be viewed in that context - while they are useful in understanding the opinions of voters in the seats that will decide if the Conservatives will get an overall majority or not, you can't compare them to the findings in national polls, because they are by definition polling areas that are more Labour to start with.