If the Yes campaign had its very own Project Fear, at its heart would stand four letters - Ukip.
This seems an odd assertion. After all, the Scottish branch of Ukip is beset by civil war, the party has no MSPs or councillors and at the Cowdenbeath by-election earlier this year, it achieved a miserable 3% of the vote - even less than the 5% it achieved in Scotland in the 2009 European elections.
In England, of course, it's a different story. Ukip is neck-to-neck with Labour in the polls. It's forecast to win over 30% of the votes in the European elections down south.
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It does seem bizarre that Ukip can attract so much support. It has only three clear policies - exiting the EU, opposing all immigration and, thirdly, representing a putrid mix of the nastiest, most poisonous aspects of the British psyche.
Then there's Nigel Farage, the Ukip leader who hardly dares show his face north of the border. A national joke surely? A party leader who dismisses his party's manifesto as "a load of late-night ramblings", claiming he's never read it anyway. A Euro MP vehemently opposed to the financial extravagance of the EU but who hints that he's not especially careful about the expenses claims he submits to Brussels.
An anti-immigration extremist who condemns foreigners taking British jobs but employs his German wife as his secretary. A politician whose response to any critique is a guffaw, a puff on his fag and a slurp of his ale.
How do he and his party get away with it?
It's convenient to put forward the explanation that Ukip is just a protest vote. After all, who pays any attention to the European parliament anyway? When domestic elections come around, voters will return to 'mainstream' parties.
I wouldn't be so sure of that. Ukip represents a fair range of the political spectrum in England.
It already has the racists in the bag. Those who blame Johnny Foreigner for every ill can be ticked off as well. Not for nothing has Ukip been called 'the BNP in blazers'.
It's also assured of the support of the fox-hunting fraternity, the retired Colonel Blimps and the Disgusteds of Tunbridge Wells and elsewhere who long for the day when Downton Abbey returns to our television screens as a factual documentary.
But there are plenty of English people turning to Ukip who are neither racist nor reactionary.
It's easy to understand discontent with the EU. What started out as a mission to create a just, democratic Europe has lately appeared to be little more than a bureaucratic agency for fostering globalisation and feather-bedding big business.
It's easy too to understand the frustration of communities which have seen a huge influx of foreigners over the last decade or so. There's nothing wrong with immigration per se, but local services need to keep in step.
With the UK dismantling state and local government planning and savagely cutting public spending, not surprisingly the benefits immigrants bring are submerged under protests about overwhelmed schools and surgeries, depressed wages and rocketing rents.
But what's all this got to do with Scotland?
The rise of Ukip is relevant to Scotland because it has the main UK parties running scared. It has pushed Labour and the Tories to the right and not just on Europe and immigration. In its election leaflets in the north of England, Labour is appealing to voters not to back Ukip. It clearly fears disenchanted Tory voters will never return to Labour.
Ukip has also given voice to the disenchanted in the English regions who want their areas to be prioritised before even more devolution is given to 'subsidised' Scots. While Ukip - which proposes replacing MSPs with Scottish Westminster MPs - remains a force, don't expect Labour or the Tories to be hastening towards more powers for Edinburgh in the event of a 'No' victory in September.
Most importantly of all, Ukip gives voice to a set of political sentiments still strong in England - deferential, class-ridden, self-interested, anti-foreigner. Such attitudes exist in Scotland but they are much weaker and more marginal than in England. In a YouGov survey, over 20% of English voters identified Ukip as the party which "best stands up for the interests of England." Only 1% of Scots thought it best for Scottish interests.
We can be sure Ukip won't fade away in England come the end of May. It plugs into more than just concerns about the EU.
Its strength in England is a rebuke to the revisionist unionist tendencies which would have us believe there is no difference between Scottish and English political cultures. Indeed, current opinions polls show over half the English electorate supporting Tories or Ukip. A future coalition from hell perhaps?
The English don't deserve Ukip. But it's their problem to solve, their political values to sort out.
In the meantime, we Scots shouldn't feel smug. While we're tied to Westminster, Ukip can't be regarded as a joke in Scotland. Its malign influence is much too scary to be a laughing matter anywhere.