ComRes have published a new poll of voting intentions in LD-Con seats in the South West for ITV. Full details are here. The topline figures are CON 44%, LAB 13%, LDEM 26%, UKIP 10%. Given these are all seats that the Liberal Democrats won in 2010 this is a huge turnaround – in 2010 the Lib Dems had an overall lead of 8.5% over the Tories in these seats, now they are 18 points behind, a whopping great swing of 13 points. If there was a uniform swing of this scale across these seats the Lib Dems would lose the lot.

Depressing for the Lib Dems, but wholly at odds with previous polling evidence in these seats. Lord Ashcroft has polled Lib Dem held seats pretty comprehensively, so we actually have constituency polls in 12 of the 14 seats included in this sample, and they paint a very different picture. Compared to the 13 point LD>Con swing in the ComRes poll Lord Ashcroft found an average LD>Con swing of about 4 points.

The difference between these two sets of polling is much larger than can explained by margin of error – they paint a genuinely contradictory picture. If ComRes are right the Lib Dems have collapsed in their heartland and face wipeout, if Ashcroft are right they are holding up against the tide and should retain around half those seats.

Explaining the difference is a little harder. It could, of course, simply be that public opinion has changed – some of Ashcroft’s polling was done late last year… but most of the Lib Dem collapse in support came early this Parliament, so this doesn’t ring true to me. Looking at the rest of the methodology both polls were conducted by telephone, the political weighting was much the same, the turnout weighting not vastly different.

My guess is the difference is actually a quite a subtle one – but obviously with a large impact! Both Ashcroft and ComRes asked a voting intention question that prompted people to think about their own constituency, candidates and MP to try and get at the personal and tactical voting that Lib Dem MPs are so reliant upon. However, looking at the tables it looks as though ComRes asked that as the only voting intention question, while Ashcroft asked it as a two stage question, asking people their national preference then their local voting intention. The results that ComRes got in their constituency question are actually extremely similar to the ones that Ashcroft got in his initial, national question.

This sounds weird, but it’s actually what I’d expect. When I first wrote the two stage voting intention question back in 2008 my thinking was that when people answer opinion polls they want to register their support for the party they really support, not a tactical vote or a vote for their local MP… and even if you ask the question slightly differently, that’s the answer you are going to get. If you really wanted to get people’s local voting intentions, you needed to first give them the opportunity to express their national support and then ask them their local support.

That though, is just the theory. As I’ve written before when writing about constituency polls of Lib Dem seats and marginal polls of Lib Dem battlegrounds, we don’t really have the evidence from past elections to judge what the most accurate methods are. Hopefully we’ll get enough different constituency and marginal polls over the next three weeks to give us the evidence to judge in the future.

Meanwhile tonight’s YouGov poll for the Sun has topline figures of CON 34%, LAB 35%, LDEM 8%, UKIP 13%, GRN 5%