When you sit down to do a marginals polls one consideration is where you draw the line: what is a marginal? The thing you want to avoid is under or overshooting the real battleground – the risk is that you poll lots of seats that need a swing of up to 5% and find a swing of 10%, enough to win lots of seats you didn’t bother polling.
That’s the sort of thing that’s happened in the third of Lord Ashcroft’s three sets of marginal polls – full details here. He polled the four most marginal LD -v- Lab seats, Norwich South, Bradford East, Brent Central and Manchester Withington. These need a swing of up to 2.1% to go from LD to Lab, which unsurprisingly Labour get easily. The average LD => Lab swing in these seats was 15%, confirming that the Lib Dems are doing much worse where they are up against Labour and easily enough to unseat almost all Lib Dem MPs with Labour in second place. In practice of course we can’t actually be that confident that voters in a tight LD-Lab marginal will behave the same way as in a seat where the Lib Dems have a 20% majority, so it’s a bit of a shame Ashcroft didn’t include some more challenging LD-Lab fights like Cambridge, Hornsey & Wood Green or Bermondsey.
An interesting thing to note is that the Greens are doing notably well in a couple of these seats. In Norwich South they are in second place on 20%, but that was one of their target seats anyway, they are also doing well in Manchester Withington, up 8 points on 10%.
While it’s hardly a LD-Lab battle, Lord Ashcroft also polled Brighton Pavilion, Caroline Lucas’s seat. Voting intentions there with changes from the general election are CON 18%(-6), LAB 33%(+4), LDEM 5%(-9), GREEN 32%(+1), suggesting an extremely tight race between Labour and the Green party.