Lord Ashcorft has published some new polling of marginal seats, full details here. As with the ComRes marginal poll in the week the seats polled were mostly ultra-marginal seats – in this case, the 12 most marginal Con-Lab seats, the 12 most marginal Lab-Con seats, but whereas the ComRes poll was a single sample representing the most marginal 40, these were 24 individual samples, one from each seat. Ashcroft also added two seats that are less marginal, but thought to be good for UKIP: Thanet South and Great Yarmouth.
The fieldwork for Ashcroft polls was done between the 31st March and the 18th May. During that period the average Labour lead in the national polls was about 3.5 points: that’s the equivalent of a uniform swing of 5.25%. The average swing in the twelve CONSERVATIVE ultra-marginals seats was 5.5%, The average swing in the twelve LABOUR ultra-marginals was 6.5%.
That means that in contrast to the the ComRes poll in the week, the swing from Con-to-Lab in Conservative ultra-marginals is pretty much in line with the national swing, a fraction of a percentage point better for Labour compared to the national figure. In Labour-held ultra-marginals the swing is a little larger, which is what we’d expect to find (parties do a little better in seats they hold due to the incumbency effect of the local MP).
It’s not a very exciting finding – swing in Conservative marginals not vastly different to other seats – but it’s one that gives me some confidence in the poll. The reality is that come general elections marginals as a group are not usually vastly different to other seats. The swing is sometimes a little bigger or smaller, new incumbents normally do a little better, but the contrast isn’t normally vast.
While I excluded them from the sums above (as they were selected because they were unusual, so would have skewed the sample) I should comment on those two extra seats polled – Thanet South and Great Yarmouth. Both were chosen because there was an expectation that UKIP would be doing well, and in both cases it proved to be true – both had them in a very strong third place, with 28% in Great Yarmouth and 27% in Thanet South. Their strongest performance though came in a seat that was part of the normal sample of ultra marginals – Thurrock, where Ashcroft found them at 29% and in second place behind Labour. Thurrock was also one of the seats where UKIP did extremely well in Thursday’s local elections.
UPDATE: Actually I’ve just spotted that the fieldwork in the Tory held seats was done earlier than the fieldwork in the Labour held seats. So comparing the swing in Con-Lab seats to the swing in national polls at the time the polls were done shows no difference at all (both show swing of 5.5%). Comparing the swing in Lab-Con seats to the swing in national polls at the time those polls were done shows Lab doing about 1.5 points better in seats they already hold.