If "SNP sources" are saying a second referendum cannot be held until Yes have been on 60% for a full year, they have utterly lost the plot

There. I've said it. As you know, I've had concerns recently that the SNP might be preparing to make itself the hostage of the polling industry. I've also been a tad worried that the party might be listening to siren voices like that of John Curtice, who if memory serves me right once suggested that a second referendum shouldn't be held until Yes had topped 60% in the polls for more than two years. (Although I naively thought the latter concern was a lesser one, because it was self-evidently such a ludicrous proposition.) My fears eased during the conference, because no specific target figures were mentioned - it was simply stated that a significant and sustained shift in public opinion would be a potential trigger for a referendum. That left plenty of leeway for common sense interpretations as the next parliamentary session progresses.

But now we suddenly learn that an "SNP source" has told the BBC that (barring Brexit) the party does not intend to hold a second referendum until Yes support has been at 60% for twelve months. That's almost as extreme as Curtice's suggestion. I hardly even know how to begin explaining just how daft this 'strategy' is.

1) The strategy doesn't inhabit the real world. I'm sure we all wish Scotland was like Norway in 1905, ready to euphorically link hands and vote for independence almost unanimously. We're not that fortunate, but everything is relative, because we are lucky enough (unlike civic nationalists in Wales and Northern Ireland) to live in a country where a majority is at least open to the idea of backing independence. However, that potential Yes majority is a relatively narrow one. Probably 40% of the population have such a strong emotional attachment to Britain that they are totally unpersuadable under any realistic circumstances. The 60% strategy therefore insists that practically every single person who is even theoretically persuadable should be firmly in the Yes camp for a full year. That is simply not going to happen.

Why should we even think that is a problem? In a democracy, 50% + 1 is sufficient. Obviously there are legitimate concerns about slippage in support once a referendum is actually underway, but if you ever want independence to happen, you have to work with what you've actually got, and what can realistically be achieved. 60% will not be attainable in anything close to the foreseeable future. Unless the SNP leadership is just cynically cooking up a mechanism for putting off a referendum indefinitely while allowing the grassroots to live in hope, the unofficial target figure should be considerably lower - probably more like 55% at the absolute most.

2) The strategy is driven by an extremely unsophisticated understanding of how public opinion is likely to shift during a referendum campaign. The Curtice suggestion seemed to be based on the hoary old myth that voters always swing dramatically back in favour of the status quo as a referendum approaches, and that you therefore need a mind-bogglingly enormous buffer before you can risk firing the starting gun. But as we've discussed before on this blog, the more important the issue being decided is, the less likely that this supposedly "iron law" will apply. There are several examples around the world of crucial constitutional referendums, not least our own vote last year and the Quebec referendum of 1995, in which there was actually a swing against the status quo over the course of the official campaign period. The reason is simple enough - the initial Yes vote is less likely to turn out to be 'soft' if it's an issue that people care about and have already thought about in quite a bit of depth.

So quality of support matters, not just quantity. I'd rather go into a campaign with a 55% Yes vote comprised of 51% 'firm' supporters and 4% 'soft', than have a 60% vote comprised of 40% 'firm' and 20% 'soft'. Presumably the reason for stipulating that the Yes vote has to be implausibly high for a full year is the belief that support can be assumed to be firm if it holds up for a long time, but that's entirely wrong-headed - look, for example, at the sustained lead Neil Kinnock had in the polls in 1989-90. A soft vote can very easily look consistent. You simply can't get a sense of how deep your support is by looking at headline voting intentions only - for the really useful information you have to drill down into the results of supplementary polling questions.

3) The strategy is looking for absolute safety which will never be available, even at 60%. If you catastrophise by looking at past examples of referendums where the Yes campaign has haemorrhaged support as polling day approached, the only rational conclusion to draw is that no level of Yes support is ever enough. I'm afraid we just have to accept that the electorate tends to behave unpredictably in referendum campaigns, and that it will never be possible to know the result in advance. We do not, thank God, live in North Korea.

4) The strategy hands far too much power to polling firms and their potentially questionable methodology. During the referendum campaign, there was a huge gulf between Yes-friendly and No-friendly pollsters, which simply couldn't be explained by margin-of-error effects. To this day, we have no way of knowing which camp was more accurate, because all of the results suddenly converged at the close of the campaign. But if, for the sake of argument, the Yes-friendly firms were closer to the truth, that means the methodology of No-friendly firms (most obviously YouGov's notorious "Kellner Correction") was artificially dragging down the average Yes vote by several percentage points. There's no reason to suppose that can't happen again, because a big divide has already opened up again between different pollsters (albeit we now have a completely different line-up of Yes-friendly firms).

So I can't deny it - I'm extremely disheartened by what we've heard today. The only consolation is that nobody in the SNP leadership has yet mentioned a target figure on the record, so there's still scope for them to quietly revise their position over the coming years as they realise how crazy it is. That might happen, for example, when Yes has been consistently hovering at 54% for a year or two, and people start scratching their heads and wondering aloud why that isn't enough.