That old air of haplessness still hangs around Sir John Major.

Even as an elder statesman, he behaves like someone who has joined all the dots and still can't make out the picture. To put it another way: if the former Prime Minister can be so wrong about the past, what price his view of the future?

When he is not simply wrong, Sir John is forgetful. Yesterday, he was back with another of his periodic efforts to save the Union, warning that an SNP arrangement with a minority Labour government would see Ed Miliband subjected to a "daily dose of political blackmail". Sounds awful.

It didn't seem quite so terrible to a Tory leader called Major when he found himself clinging to office at the end of 1996 thanks only to the votes of Ulster Unionists (UUP). Did Sir John forget about that, and all the deals he sanctioned while his party seethed with the complaints of esteemed colleagues he called - in jest, no doubt - "bastards"?

Today, he reckons that a Labour-SNP understanding is a "frightening prospect", threatening a "weak and unstable" government "held to ransom". His shorthand word is "mayhem". The politician who gave us forced withdrawal from the European exchange rate mechanism wouldn't know anything about chaos, of course. The leader of the administration that added "sleaze" to the political vocabulary is our expert on sound government.

But let's not forget the UUP. They cut their deals with Sir John's tribe without public hand-wringing on his part. Let's not forget, too, that the Democratic Unionists (DUP), Northern Ireland's successor party, launched their manifesto yesterday with a straightforward price list. Their "pivotal" support is available if conditions are met. So does Sir John think David Cameron should reject the DUP? If so, he forgot to mention it.

Equally, the elder statesman did not manage to grumble too loudly in 2010 when a handful of Liberal Democrats bargained their way to the Cabinet table. He had no objections in principle - practice being another matter - in the days when Labour in Scotland were returning 49 (1992) or even 56 (1997) MPs. Amid hysteria from the Tory papers and quavering panic from Sir John, the constitutional facts survive: nothing prevents these outcomes. Form a majority, any way you can, and you govern.

So what animates Sir John and encourages London journalists to vent the kind of abuse David Hume would have recognised in the 1760s? Try this from the former Prime Minister: "The SNP is a real and present danger to our future". Shorn of melodrama, this is the familiar formula: a party dedicated to ending the Union of parliaments should have no say in Westminster government. But what does that really mean?

Remember the constitution. The legislation creating the Scottish Parliament is now part of that body of law. Holyrood is one devolved part of the government of the United Kingdom. Yet by Sir John's logic a lawful party controlling an administration in Edinburgh must be kept away from any influence, far less control, at Westminster, within that same shared UK. "Our future" is at stake.

Ever get the feeling you've been disenfranchised? After all, Sir John is telling anyone who votes SNP that their vote must never count for anything. He might as well have called for the prohibition of Nicola Sturgeon and her party. But unlike the elder statesman, let's not get carried away.

Tories are playing games with Labour to maximise support for Mr Cameron and damage Mr Miliband. The idea - deeply unBritish, more than a little insulting - is to frighten English voters with the prospect of the Scots. In the crazed language now seeping out of London, a moderately leftish party is "hard left" as well as nationalist. Should you fancy supporting that you are, collectively, mad. On the face of it, nevertheless, Sir John demands no more than he can demand: gang up and freeze out the SNP.

Let's say Ms Sturgeon's colleagues do as well as some expect. Let's say there are 50 of their number on the road to Westminster. That leaves just 600 MPs from other parties who can decide for themselves what is and is not a real and present danger. In the meantime, the flood of rhetoric from Sir John and others less restrained is bound to have its effect on voters on both sides of the Border. What's notable, if it counts as old news, is how little those suffering fainting fits over the SNP care about that.

Michael Forsyth, Baron Forsyth of Drumlean, is more perceptive than his old boss. As he argues, talking up the SNP just to hurt Labour is one thing, and bad enough. It does Unionism generally no favours. But giving Scottish voters the idea that their participation in UK politics is conditional, that their choices can be ruled illegitimate if English nationalism is roused, will be lethal to the Union.

Mr Cameron and those running his campaign don't seem to mind. For some of us, that only confirms an opinion; for others, it will come as an education. This is where we stand in the Westminster scheme of things. So who wouldn't want to see an end to an old confidence trick?

Unionist parties are united in this. The LibDems say they will play no part in any arrangement involving the SNP. Alistair Darling, fresh from leading the Better Together effort, says the idea of Labour reaching any form of accommodation with Ms Sturgeon's party is "for the birds". And Mr Miliband seems prepared, at last, to agree publicly with his former colleague.

Labour in Scotland have been sketching the grand scheme for a while. Mr Miliband will simply put forward a Queen's Speech and the SNP will be invited to take it or leave it. That would be a confidence matter. The choice would be between supporting Labour, like it or not, or "letting the Tories in". And would the SNP dare?

It's not clear when a Queen's Speech became a binding five-year contract. It's not obvious that Mr Miliband understands the Fixed-term Parliaments Act, or even the use of Commons amendments. But a confidence issue does not mean "letting the Tories in". It means another general election in which the Labour leader would have to explain why he gave up the chance to - keeping the Conservatives out of office - because he would not deign to accept support.

On the parochial level, in places where Westminster leaders do not show their faces, that doesn't seem calculated to appeal to the former Labour voters in Scotland who have chosen the SNP. It won't alter the electoral arithmetic in Mr Miliband's favour if he fancies a second crack at the campaign trail. But perhaps that's just a reminder of this country's allotted place on the Westminster map of the UK.

Forming a political party - recruiting members, campaigning on the streets, arguing, winning votes - is the only democratic way to achieve a goal. Yet the moment it seems it might actually work for once, Sir John and a shrieking chorus say it can't be allowed. That says everything about them.