If they behave differently to the national polls, and if different groups of
marginal seats behave differently to one another, it's obviously a very big deal.
Not exact matches
You can see that
marginals do
behave a little differently sometimes — the Conservatives managed a better swing in their target Labour
marginals in 2010, Labour did better in those
seats where they had fresh incumbency in 2001 — but the differences aren't huge.
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.
If history is any guide we might expect the Conservatives to do a little better as most of the battleground
seats have new Conservative incumbents, but only by a very small amount — the reality is that Con - Lab
marginals do tend to
behave in pretty much the same way as the nation as a whole does.