As the national polls continue to hover within hung-parliament territory, many are pinning their hopes for a decisive victory on differential swing in the marginals. Is that a reasonable hope?
All the parties concentrate their ground-war resources on the marginals and if we suppose one side or another is doing a better job at that, then obviously we would expect a better outcome for them than the national polls predict. There are good reasons for supposing that the Conservative campaign is better financed and organised than Labour's, so that suggests a rosier scenario for Cameron.
There may be another crucial factor: people are more likely to become aware that they are living in a marginal seat as the campaign hots up; this will make them more motivated to vote, and more willing to vote tactically.
People are influenced by each other's behaviour; as we have seen a clear swing to the Conservatives, and as they benefit from a (small) overall preference among the public, we might expect increased motivation to split disproportionately in their favour.
So there are two factors that give the Tories some grounds for increased optimism. But remember, even if they are real, they are likely to be small - in my view the highest possible expectation is an added two percent of swing (i.e. an additional four percent on the lead).
