Who really knows about the "don't knows"?

The claim by SNP referendum campaign director Angus Robertson that up to 40% of those registered to vote in the independence poll may yet be reachable merits scrutiny. Even though a poll in December had the "don't knows" at 33%, recent figures suggest many fewer (between 11 and 15% of voters)are undecided.

If Mr Robertson is correct and more than twice that number are yet fully to make up their minds, more than one million voters remain effectively "don't knows". Pro-Union supporters will assert this amounts to wild over-optimism from the Yes camp. But the SNP say the figure is backed by internal polling. In addition, Mr Robertson believes most undecided voters are willing to vote Yes and are ready to be persuaded.

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For much of the campaign, it is true, polls have indicated around 20-25% of those asked expressing firm support for independence. Meanwhile around 35% are firmly against. That would leave 40% who are, at the very least, not firmly committed to one cause or the other.

It is also possible the number of committed voters is overestimated by pollsters. People have a tendency to say what they think others want to hear. Some may be responding according to party allegiances of varying strength. Others may give answers that suggest their views are firmer than they truly feel; yet more may be embarrassed still not to have a clear decision in mind. The language for such people is oddly pejorative: floaters, waverers, switherers, undecided. Regardless of choice, the language has an air of criticism. But there is no shame in being still unsure. If people are happy to "ca' canny" for now, it is a sign they are weighing the significance of the choice and are demanding to be persuaded. Such voters are important. With a high turnout predicted, neither camp can entertain complacency about their likely impact on the outcome.

If there are so many undecideds, how are they to be swayed? For the No campaign this seems straightforward. We know maximum devolution remains popular, which is why pro-Union parties are attempting to sell their ideas for this virtually daily.

It seems logical that many "don't knows" would readily vote against independence, given an alternative in the form of extra powers. But this will work only if the offer is clear, credible and perceived as genuine.

The Yes camp has a trickier balance to strike between appealing to the head and the heart. Mr Robertson acknowledges that appealing to the head is the more important. Independence can yet be won, he believes, but only if voters can be provided with the information they need. That must mean addressing the outstanding issues around the economy, currency, defence and the EU. The problem for the Yes camp is that some issues might remain unresolved, with certainty dependent, for example, upon negotiations after the vote. However many undecideds there are, the task of persuading them seems a bigger challenge for the Yes campaign.