It was not helpful of Michael Forsyth to give an interview to the Guardian criticising aspects of the Conservative campaign about Scotland.  And it would not be right for MPs with Scottish seats to continue to be able to vote on the affairs of England, Wales and Northern, as he suggests during it.  It wasn’t before the independence referendum – and it is even less so now that the Smith Commission plan is to happen.  The prospect of a weak Ed Miliband being propped up by a strong SNP force at Westminster is a real one – one which Stewart Hosie’s warning that the SNP may block the defence budget has brought to ominous life.  It follows that David Cameron has a duty to warn voters of it.

None the less, there is more to politics than simply being helpful to your own party, and Lord Forsyth’s main point is worth pondering.  The nub of it is that the long-term future of the Union is more important than this short-term election campaign – a case that ConservativeHome has itself made.  Though he doesn’t quite put it this way, he implies that while Labour is an opponent, the SNP is an enemy, since the latter wants to break the country up while the former does not.  It therefore follows that it is wrong for Ministers to build up Nicola Sturgeon’s performances in TV debates in order to knock down Miliband: the former Scottish Secretary is right about that.

It also follows that everything the Conservatives do should be consistent with preserving and enhancing the Union.  And since the best way of doing so is now a federal Britain (the pre-1997 constitutional settlement now being dead), Tory policy should aim to trump Sturgeon and Salmond’s “full fiscal autonomy” – part of the point of which is to raise Scotland’s defence revenues north of the border in order to withhold them from the country as a whole.  Instead, Scottish taxpayers should be offered to chance to raise their own domestic revenue – so no more Barnett formula – while the UK taxpayer would stump up for Scotland’s defence, foreign affairs spending, and anything else deemed to be federal business.

Some argue that the sums don’t add up.  (A case put by Nick Pearce of the IPPR.)  Others maintain that Scotland could fund much of its own spending.  (David Davis says that he discovered after the 1997 that Scotland could just about pay for its then new devolved responsibilities.) But while transitional funding might be required, there is no escaping the iron logic of federation – at least, if the Union is not to implode under the pressure of the unfairnesses of the post-Smith settlement to England in particular. The Conservative Manifesto clings cautiously to the status quo, saying that “we will implement the recommendations of the Smith Commission so that more than 50 per cent of the Scottish Parliament’s budget will be funded from revenues raised in Scotland”.  But we should be aiming for 100 per cent.

The Party leadership, struck by the campaigning impact Labour gained by deploying Tony Blair, is today deploying John Major.  A test of his speech will how it balance the tricky task of warning English voters about the dangers of the SNP tail wagging the Miliband dog with not simultaneously driving Scottish voters into the hands of the party that would break our country up – a microcosm of the problem facing the Conservative Party as a whole.  Sir John has a solid record on the issue: he warned before the 1997 election that the Blair/Brown devolution fudge would be unsustainable, and so it has proved.  But as a former Scottish Secretary – and someone who saw the same problem coming – Lord Forsyth sees the issues just as clearly.