How can this be? How can Scottish Labour voters, apparently in their tens of thousands, be contemplating voting for the SNP? So toxic has been the relationship between Scotland’s two left-of-centre parties, that to make the switch would once have been like Rangers fans popping up en masse in the stand at Celtic Park.

Yet here we are with the average of the most recent three Scottish polls showing the SNP vote on 46% and Labour on 28%. In the 2010 election Labour had 42% to the nationalists’ 20%. Six weeks is a very long time in politics, but that the tectonic plates have shifted in Scotland is surely beyond doubt.

Here we are with former SNP leader Alex Salmon popping up on the Andrew Marr show to tell his host that “if you hold the balance, you hold the power”, and suggesting how the “tartan benches” at Westminster might deal with an Ed Balls budget.

For many of us all this is a less strange phenomenon than it must appear on the southern side of Hadrian’s Wall. Like the majority of left-of-centre Scots the Labour party once seemed my natural home. But, as with many left-of-centre Scots, events conspired to change my perception.

The journey has not been painless. I was proud and privileged to deliver a funeral eulogy to the late Donald Dewar, Scotland’s first first minister and a much respected personal friend. I had, still have I hope, many friends within Labour ranks. But I was never tribal Labour; I voted for the party that best reflected my beliefs and values.

How has this changed? Let me try and count the ways. Connivance in illegal wars is a given, but a revulsion to that is shared by many UK voters of all political persuasions.

There is also a specific Scottish dimension to a conversion that has been gradual rather than Damascene.

It seems to me that Labour has failed to hold the line against a whole series of rightward-leaning innovations in health, education, defence policy, macroeconomics and, most acutely, in welfare.

At the very least I wouldn’t have expected Labour’s endorsement of tuition fees, an early involvement in NHS marketisation, an agreement to Trident renewal, and a commitment to maintain welfare cuts and austerity targets predicated on punishing the poor for the folly of financiers.

I’m not sure if some London-based media fully appreciate the extent of the philosophical divide since Holyrood set up shop. It has become a commonplace in some quarters to lazily bracket the SNP and Ukip as two sides of the same insurgency coin. Parties of protest who don’t know what they’re for, only what they’re against. This fails to take account of the SNP having been in government for eight years now, first as a minority administration.

That stint earned it enough respect to return with a comfortable majority in 2011. Of the 129 seats at Holyrood it now holds 69 to Labour’s 37. It has continued free personal care of the elderly, abolished prescription charges, and set its face against tuition fees, Trident, and private health expansion.

Equally dubious is the analysis that the SNP’s advance – it reached 100,000 members at the weekend – is merely a product of referendum fever, and that Scotland will inevitably revert to being a largely Labour fiefdom. For the decline of Scottish Labour has been a long, slow burn exacerbated by the decision to campaign alongside the Conservatives in the Better Together campaign.

For many erstwhile Labour supporters this wasn’t so much supping with the devil as throwing him a house-warming party. The new hyperactive Scottish Labour leader, Jim Murphy, is working hard to reverse the presumption that London pulls all the strings and is trying the saltire on for size.

For many Labourites it may be too little too late. Whatever side of the referendum debate they were on, there has been shared exasperation at the constant refrain about whingeing jocks enjoying special perks, despite national taxation contributions giving the lie to that charge. And there has been genuine puzzlement as to why demanding full fiscal autonomy with its attendant responsibilities could be construed as thrusting more fingers in the Treasury till.

This most unpredictable of UK elections has thrown up many ironies, not least of which is the Tories warning that voting SNP will hand Labour the keys to No 10, while Labour assures electors that supporting the nationalists will give power back to David Cameron. But there is, as that nice Mr Blair was wont to observe, a third way. Another reason for plighting your troth to a party that will not be led by a prime minister. Many people who vote SNP in May will do so for the first time in a Westminster election. Previously they would not have seen much point in dispatching a nationalist force to London.

But the 2015 election is a different animal. We are living in different times. Strange times. And, odd as it may seem, many Scots will vote SNP in order to try and help a Labour government rediscover its soul.