Come April, Sir John Major will have been saving the Union for 23 years.

It's a proud record. If a contest is ever held between the former Tory leader and Gordon Brown over the sheer number of their "rare" interventions in constitutional affairs, it will be hard to pick a winner.

It was my privilege to witness Sir John finding his noble tradition all those years ago. As a press denizen of a Tory "battle bus" on the Major Magical Mystery Tour in 1992 - they did a nice packed lunch, to be fair - I was present when the Sacred Soapbox of Smith Square materialised. A tiny legend was born.

Often enough, abuse and eggs followed. Some of those Labour crowds had a way with words and missiles. Unlike others we could mention, however, the candidate didn't grumble. In fact, he seemed to enjoy himself. He enjoyed himself a lot more on the night of April 9 when it became obvious that Neil Kinnock had blown his chances.

Sir John would later conclude that his last-minute appeal to voters to avoid "a headlong plunge into something of dire consequence for Britain" had helped to swing the battle. At a rally at Wembley four days before polling he had damned devolution and cried: "Wake up my fellow countrymen. Wake up now, before it is too late".

For the record, the SNP had three MPs when the 1992 campaign began. They still had just three by '93. Sir John was not reassured. Heading for defeat in 1997, he revived his theme ("There are 72 hours in which to save the Union"). The SNP, building its share of the vote, returned just six of Scotland's 72 MPs that time. Labour, getting ready to "kill nationalism stone dead", had 56 tribunes.

As the years have passed, Sir John has come to resemble a greatest hits act on a perpetual farewell tour. In 2007, he warned that the Union was "unstable" and likened the Barnett formula to "Scot-geld". In 2011, he returned to his "Battle of Britain" with the startling - to Unionists - assertion that Edinburgh should have power over everything save defence, foreign affairs and the economy. Devo-max was the way to save the Union, though only if Barnett was abolished and the number of Scottish MPs cut.

Last autumn, just before three panicking Westminster leaders trekked north, Sir John was back to the old script. A Yes voted spelled disaster for all of Britain, but particularly for those Britons attached to their nuclear deterrent and UN Security Council seat. Scotland would toil, meanwhile, but the Welsh, sure as fate, would be next to the exits.

Given all that has followed the referendum, you might have thought a period of silence from Sir John would be welcome in Unionist quarters just now. Not a bit of it. He might resemble the Ancient Mariner of constitutional debate, but yesterday morning he was back in The Daily Telegraph, foretelling doom if Ed Miliband does not rule out a post-election deal with the SNP. It would be "a lethal cocktail for the United Kingdom". The fact that Labour's leader has not turned his face against the prospect is "shameful".

Well, a Tory would say that: the party's chance to govern hangs in the balance. But a Unionist capable of rising above party - isn't that the general idea? - would say it too. Only last autumn Labour was throwing itself on the wire to stop a Yes vote and, by extension, the SNP. Any succour to nationalism was anathema. Those preparing to vote for independence were dupes or crypto-fascists intent on destroying the UK. The UK, moreover, that Labour now hopes to govern.

For Mr Miliband, this ought to be personal. Lord Ashcroft and other pollsters have delivered a single message: Labour in Scotland has been wounded grievously, perhaps fatally, in the referendum's aftermath. Could the party's leader contemplate rewarding those who have wrought such havoc, who mean him no good, who regard even an informal deal as a milestone to independence by other means?

Of course he could. Other candidates for even a loose alliance are fading by the day. Many of Mr Miliband's Scottish Labour MPs will not be around to complain on May 8. Jim Murphy can recite his "vote SNP, get Tory" line as often as he likes, but that, like the rest of the policy-a-day headless chicken routine, isn't working. Something fundamental has happened and anything Mr Murphy has to say is close to irrelevant. Mr Miliband surely realises as much.

So he rules nothing out. He might yet surprise us all when he addresses his Scottish party's one-day conference in Edinburgh today. He might put Sir John's sacred cause before his chances of becoming Prime Minister. It's more likely that Mr Miliband will start to paint a few pre-negotiation red lines of his own. If the SNP dissent, they can take their chances as abstentionists. That might look - so some in Labour still hope - a less than impressive version of "standing up for Scotland".

Nevertheless, the game is afoot. After all, what was it those No campaigners said? "Please don't go"? This is what follows: for now, we stay. Our Westminster choices are becoming abundantly clear. Are they deemed illegitimate choices by Sir John and the ever-excitable band of Unionists stumbling towards the mirage of tactical voting? So it would appear. The Major thesis holds that the democratic choices of Scots are not fit for polite Westminster company.

A deal still seems like a good bet. That might suit Nicola Sturgeon in her drive against austerity, but it doesn't square every circle for the SNP. When - or rather why - did Labour become acceptable, even at arm's length? Aren't these the red Tories who worked hand in glove with blue Tories during the referendum campaign? Isn't Labour indistinguishable from the Conservatives with whom Nationalists will have no truck?

Ms Sturgeon is being pragmatic, no doubt. She would probably explain that she harbours no affection for Labour, but calculates that the embers of progressive politics still flicker within Mr Miliband's party. Nevertheless, strategic considerations do not quite cohere with election campaign tactics. It's one thing to supplant Scottish Labour in order to remedy Labour's failures. Any kind of Westminster deal would still be a deal with the enemies of independence.

And then there's Trident. The SNP has not budged from its principles, but if a Guardian interview with Ms Sturgeon is anything to go by, their application will fall short of decisive. In an "issue by issue" deal, it seems, nuclear weapons are one issue that can be set aside. The SNP will not vote for the renewal of the hideous system - of course they won't - but they will not attempt to force Labour to follow suit.

There might be time enough for that. Mr Miliband could find himself depending on the SNP for a long time. If Labour is humiliated in Scottish elections in 2016 the redrawing of the electoral map will begin to have a permanent look. History, to which Sir John Major and his admirers are fading footnotes, will commence a new page.