What makes the intervention important, and what elevates it above the standard squabbling of Scottish politicians, is that it damages the central plank of the pro-Union Better Together operation just at the moment when the campaign is already under intense pressure from Mr Salmond. Some on the pro-Union side are starting to fear that the race – which could lead to the break-up of the United Kingdom – could be tighter than they once thought. And it enables those running the Yes campaign to tell voters that anti-nationalist warnings are bogus.