Hamish MacDonell: Chancellor's assault has touched a raw nerve

When George Osborne was given control over the strategy to fight the Scottish Nationalists, it was seen by many as the beginning of a battle which would pit two of the best political operators in the country against one another: Mr Osborne and Alex Salmond. But, for the past year and a half, the Scottish First Minister has had the field pretty much to himself. The Chancellor has had his hands full elsewhere.

Last night's assault on one of the fundamental principles of Mr Salmond's vision of an independent Scotland showed that Mr Osborne has, at last, decided to join the fight.

The Nationalists are likely to argue that Mr Osborne has no authority to tell an independent Scotland it cannot stay in a sterling zone – and they may well be right. Decisions of that nature are probably the preserve of the Bank of England. But that is unlikely to deflect from the core of the Chancellor's argument.

Mr Osborne articulated something that has been gnawing at the Nationalist cause for the past few months: the suspicion that an independent Scotland tied to sterling wouldn't really be independent at all. If interest rates are set by the Monetary Policy Committee of the Bank of England and monetary policy is likewise dictated by London, probably with an eye primarily to the economy of the South-east of England, what independence would Scotland have?

But more than that, the euro crisis has shown up the fundamental weakness of fiscal union without political union, as countries across Europe have suffered because they have a currency which is overvalued for their needs and they are unable to print quantities of their own currency to keep their economies afloat. Mr Osborne's foray into the independence debate may be dismissed automatically by Mr Salmond, but it may well kick-start a public debate about one of the core assumptions of Scottish independence. Mr Salmond's vision has always been more about the heart than the head. Recently though, he has tried to balance up the argument by confronting such thorny issues as the currency.

But this is where he has weaknesses, and the unionists know it. The Nationalists will never admit it, but Mr Osborne touched a raw nerve last night.