Swing table yougov

The above chart is one of the most crucial of the election – it looks at how 2010 voters of each party have variously chosen to stick with their chosen side, or to switch elsewhere. Rather than normal polls, which just show headline figures and leave us to guess at the swings or churn among the parties, this looks under the skin.

There are some trends of which we’re well aware – the fact that the largest single source of UKIP votes is former Tory voters (39 per cent), for example, or the way in which ex-Lib Dems are splitting more to Labour than to the Conservatives. It certainly confirms the importance of winning over UKIP voters, who still prefer Cameron as Prime Minister, in the final reckoning.

But for me it is the “Don’t Know” column which particularly stands out. They’re rarely discussed, I suspect because they’re normally excluded from the headline polling numbers, but they are a significant share of the vote – more numerous than UKIP at the moment, it seems (at least among the ranks of 2010 voters which this poll studies – UKIP do claim to have a decent chunk of supporters who didn’t vote at all last time round).

28 per cent of the Don’t Knows are former Conservatives (worth 3.5 percentage points in total in the poll). Between them they are serious potential well of support – and squeezing them will be as important as squeezing UKIP. The question, as Stephan Shakespeare highlights over at YouGov) is whether the arguments that woo one group will also woo the other. Are those former Tory supporters in the Don’t Know column wavering between blue and purple, in which case the answer is yes, or are they at the opposite end of the Tory spectrum?