Shortly before the general election of May 1979, James Callaghan was thinking aloud to his adviser Bernard Donoughue.

"You know there are times, perhaps once every thirty years," said the Prime Minister, "when there is a sea-change in politics. It then does not matter what you say or what you do. There is a shift in what the public wants and what it approves of."

"I suspect", concluded Sunny Jim, "there is now such a sea change and it is for Mrs Thatcher." He was right about the Iron Lady but wrong about the timespan - sea changes in British politics seem to occur every 16 or 17 years rather than once every three decades.

In, that is, a UK context. There was another, of course, in 1997, when New Labour swept all before it in much the same way the SNP is doing now. Naturally, they vary in size, but every time there is a discernable shift in public opinion.

This Thursday's election looks likely to represent another sea change, albeit one primarily affecting the north and bucking the usual timescale. I keep reminding myself that between the elections of 2005 and 2010 not a single Scottish constituency changed hands; now some polls suggest the very opposite will be true.

For Ed Miliband, who's actually had a pretty good election campaign, Scotland must resemble a recurring nightmare. In 2011 every constituency he visited fell to the SNP, while at this election the Scottish sea change has basically removed (whatever the SNP say) the slim prospect of his party securing an overall majority.

I keep hearing people say with utter certainty that Jim Murphy was the wrong person to lead the Scottish Labour Party leader and John McTernan and Blair McDougall the wrong choice of strategists, but that is too easy a criticism unless one points to the right leader and an alternative strategy. As Mr Callaghan realised back in 1979, once the sea change becomes apparent, "it then does not matter what you say or what you do".

Over the past few months Scottish Labour has said and done many things, often contradictory things, and sometimes entirely contrary to what was said and done a year or so ago. Mr Murphy cannot be faulted for zeal, dedication or fortitude - all of which he channeled into the speech of his life at a Labour Party rally on Thursday night - but it doesn't matter, for he had lost the electorate's ear even before he stepped up to the plate.

Mr Murphy brings to mind Michael Forsyth in the dying days of John Major's troubled government: donning a kilt, charming the press and rushing around Scotland trying to atone for past sins. Not that this campaign has found the Scottish Conservatives in a markedly better place. Ruth Davidson has done almost everything right - generally hard-to-impress Lobby hacks even suggested she was more impressive than the UK party leader - but the Tories have been denied a voice since 1997.

In an election that saw the UK Conservatives weaponise Scotland against its opponents south of the border, no amount of creative photo-calls was going to save its northern strategy. Having tried for years to convince voters that it wasn't, as many of them still suspect, "anti-Scottish", David Cameron - or rather Lynton Crosby (whose reputation as an election winner continues to mystify) - has hung them out to dry. One might suggest a name change is long overdue, for the "Unionist" appendage no longer seems accurately to reflect its political ideology.

There is, of course, a difference between dissing the SNP and dissing Scotland; after all, Nationalists have spent decades talking about the Conservatives in similarly histrionic terms, but the distinction inevitably got lost in the heat of election battle. Desperate to recover the Little Englander (UKIP) vote, the Prime Minister promised a meaningless annual audit of the Scottish Parliament as well as the profoundly un-Unionist policy of English Votes for English Laws.

Only the Liberal Democrats have offered any sort of constitutional middle way (federalism, natch) but they too lost the right to be heard back in 2010, chiefly because of a manifesto U-turn (not increasing tuition fees) no worse than that broken by the SNP in 2007 (its pledge to eradicate student debt), all the more tragic given that by most yardsticks Nick Clegg is the brightest and best of what Alex Salmond likes to call the "Three Amigos".

The Scottish National Party, as ever, has fought an impressive campaign, especially Nicola Sturgeon - undeniably its chief asset - not just in Scotland but across the UK. Her breakthrough in that latter respect remains impressive, although of course the First Minister was always going to find it easier than three middle-class, white males to shine in the election spotlight. Campaigns can be tedious things, thus good performers are bound to stand out. In 2010 it was Mr Clegg, five years later it's been Ms Sturgeon.

Indeed the new SNP leader's strengths have simply served to highlight her predecessor's weaknesses. Can one imagine Alex Salmond emerging from two leaders' debates the most popular politician across the UK? The former First Minister, meanwhile, seems destined to grace the green benches once more; the House of Commons was his first love, but his position within the "Westminster system" shall require careful management by his successor.

Yet beneath the admittedly impressive style, the SNP layered contortion upon contortion when it came to the substance, cheerfully aware that no matter how often pedantic columnists called them out it wouldn't make the blindest bit of difference. Labour were derided as "Red Tories" while informed they'd form the major plank of an "anti-Tory" block of MPs; the SNP manifesto promised to get rid of austerity when in fact the numbers pointed to mitigation rather than abolition.

And in the course of the campaign it became increasingly obvious the party had overplayed its hand when it came to potential "influence" in the next Parliament. With its 1980s logic of "no mandate" banished, Ms Sturgeon repeatedly stressed that a majority in the Commons was all that mattered. A majority, however, is not the same as confidence, and if Labour were to form a minority government its ability to call the SNP's bluff would be considerable.

The only true "influence" comes from being prepared to vote with both main parties, which the SNP has done in the past but apparently no longer. In reality, however, any outcome later this week is potentially good for the Nationalists: locked out of power they'll simply assume the moral high ground and argue that Scotland has, once again, been ignored. In other words, the worst possible scenario is the one they claim to want.

But legitimacy cuts both ways, and if a minority Tory government emerges from the tsunami - as this column once predicted - then the "doomsday scenario" anticipated in 1987 and 1992 will have finally come to pass, requiring new weather-related superlatives to capture its significance. No one said political sea changes had to be tidy.