Apart from asking people how they currently think they might vote next year, today’s ICM/ Scotland on Sunday poll also returns to an aspect of the role that the devolution max debate might play in determining the outcome of the independence referendum. This issue was first addressed by aYouGov/Devo Plus poll a fortnight ago. It tried to find out how many people might switch from Yes to No if they were convinced that a No vote would be followed by more devolution. In contrast, the ICM poll has attempted to ascertain how many people might switch from No to Yes if they were to come to the conclusion that there would not in fact be more powers.

Given the No side are currently ahead, the crucial question for them would appear to be how far its lead might be endangered by a failure to convince people that there would be more devolution, and not whether Better Together might enjoy an even bigger lead if they were to make a credible offer.

As previous polls have found, ICM’s poll finds that so-called ‘devo max’ appears to be relatively popular. As many as 59% agree that the Scottish Parliament should become primarily responsible for taxation and welfare benefits, the two principal areas of domestic policy that are still reserved to Westminster, while only 28% think there should be no further change to Holyrood’s powers and responsibilities.

Many of those who say Holyrood should become responsible for taxes and welfare are, of course, independence supporters. But at the same time No voters are divided between 48% who are happy with the status quo and 41% who would like more devolution.

When asked what they might do if they became doubtful that more devolution would happen, most of the group of No voters who back the idea in fact say they would still stick with a No vote. But 15% say they would switch to Yes while another 12% say they are not sure what they would do.

If such a movement were to occur it would be enough to change the tenor of the campaign. If all those who said they would switch to Yes were to do so, while those who are unsure what they would do were to abstain, then the 40% Yes vote in today’s ICM poll (after excluding the Don’t Knows and Won’t Says) would rise to 45%.

The Yes side appear to be placing great store at winning over the support of those who would like more devolution but are currently inclined to vote No. Evidently many of these voters will not be won over that easily, and on its own the tactic looks unlikely to be sufficient to bring Mr Salmond victory. But it looks as though there might just be enough voters who could switch sides for the Better Together campaign to want to think seriously about the promises it should make about more devolution. For if its promises come to lack credibility in voters’ eyes, the referendum race could become a little too close for the No side’s comfort.

About the author

John Curtice is Professor of Politics at Strathclyde University, Senior Research Fellow at ScotCen and at 'UK in a Changing Europe', and Chief Commentator on the What Scotland Thinks website.