Before last year's independence referendum, there was a broad consensus among unionists. A defeat for the Yes campaign would rob nationalism of its momentum. With the SNP humbled, Scottish politics could return to a more stable dynamic: Labour dominant at Westminster and – at the very least – competitive at Holyrood.

Jim Murphy's election as Scottish Labour leader was the first sign that normalcy was reasserting itself. In contrast to his underwhelming (and overwhelmed) predecessor, the East Renfrewshire MP was a substantial politician. As a member of the shadow cabinet and former minister in both the Blair and Brown governments, he had the experience to match Nicola Sturgeon.

Murphy himself encouraged this view. Despite a series of polls showing the SNP with an unprecedented double-digit lead in terms of Westminster voting intentions, he confidently predicted Labour would retain every one of its 40 Scottish seats at the general election in May. Indeed, as recently as two weeks ago, Murphy derided the nationalists for their apparent complacency. "I’m just astonished by how quickly they’ve run out of ideas", he told the website BuzzFeed. "[They are] sluggish, lethargic, and off the pace".

But any lingering hope that the old rules still apply in post-referendum Scotland evaporated last Wednesday with the publication of Lord Ashcroft's constituency surveys. Scottish Labour's urban fortresses are crumbling in the face of a sustained SNP surge. Dozens of once unassailable Labour seats now look set to switch from red to yellow in three months' time.

It's difficult to exaggerate the significance of Ashcroft's findings. The SNP's best result (so far) at a UK general election came in October 1974, when the party won 11 seats and 30 per cent of the vote. That was 41 years ago. Scotland was in the first blush of an oil revolution, its industrial economy was grinding to a halt and nationalism remained a relatively novel force. Since then, the SNP has been repeatedly squeezed at Westminster. In 2010, Alex Salmond set a target of 20 SNP MPs. He ended up with six. The fact that 20 now seems like a modest ambition for the former first minister illustrates how dramatically the Scottish political landscape has changed.

Some commentators think Scottish Labour's collapse is the result of a) its decision to cooperate with the Tories as part of Better Together during the referendum and/or b) its failure to stop the SNP positioning itself as an anti-Westminster insurgent. Others see it as symptomatic of Ed Miliband's weak leadership. "Labour’s credibility in Scotland rests on whether it is a plausible government in waiting", argues The Guardian's Martin Kettle. "If Scots believe that Labour will form a government, the Labour vote will remain reasonably solid".

All these things factor into the mix. But Scottish Labour is suffering from a much deeper – and to some extent self-inflicted – process of attrition. The process began in the mid-1990s, as Blair and Brown exchanged Labour's post-war interventionism for a programme of liberalised markets and finance-led growth. In response, Salmond carefully manoeuvred the SNP into the vacant left-of-centre space. The strategy worked. Research from the period confirms that Scots started to view the SNP as a progressive alternative to Blairism.

The first big break came at the 2003 Holyrood election: Labour lost 250,000 constituency and 225,000 list votes, the single biggest fall in its vote share at any point over the devolutionary era. Left-leaning voters – notably Catholics, trade unionists and public sectors – bled-away from Labour, moving first towards smaller, more radical parties such as the Greens and the SSP, and then towards the SNP, resulting in ever-larger electoral gains for Salmond.

The decisive rupture occurred last year, when traditional Labour strongholds such as Glasgow, North Lanarkshire and Dundee rejected the party line and voted in favour of independence. Appalled by the flaccid conservatism of the Better Together campaign, large numbers of working-class Scots (although by no means all) embraced constitutional radicalism. On 18 September, Scottish Labour won the referendum and saved the Union, but it did so at the cost of Labour Scotland.

The only appropriate term for what is happening to Scottish Labour is "Pasokification". Just as Greek voters abandoned PASOK, the established party of Greek social democracy, for Syriza, so Scottish voters are abandoning Labour for the SNP (albeit more gradually and under less acute conditions). Clearly, neither Salmond nor Sturgeon is Alexis Tsipras, and the current Scottish government can be frustratingly cautious in its approach to reform. But the overarching trend is unmistakable: in Scotland, the old left hegemony is breaking down and a new one forming in its place.

Murphy has a two part plan to tackle this crisis. First, he intends to strengthen Scottish Labour's patriotic and working-class identity, hence his headline-grabbing pledge to use the revenues generated by Miliband's proposed tax on mansions (the bulk of which are located in London and the south east) to hire 1000 more Scottish nurses. Second, he aims to remind people that the SNP is – and has been now for the best part of eight years – a party of government. "They’ve managed to have this kind of dual identity, of being the government and the opposition", he says. "But in the public’s mind I want to put the SNP where they’ve rarely been, which is responsible for their own mistakes."

The irony, however, is that Murphy himself embodies many of Scottish Labour's most egregious mistakes. Like other talented Labour Scots, he overlooked Holyrood in order to build a career at Westminster. Until late last year, he was a champion of Blairite modernisation. He is an outspoken Atlanticist who supported the Iraq war and wants Trident – the bête noire of progressive Scotland – renewed in full, whatever the cost. Worse yet, he campaigned alongside Tory activists at a grassroots level to deliver a No vote in September. One way or the other, Jim Murphy is tied to everything currently working against Scottish Labour's long-term chances of survival. That's not to say it won't survive the upcoming election. The polls will narrow between now and May, as the prospect of a Tory victory consolidates Labour's core vote. But beyond that, who knows? Scotland simply, almost dispassionately, seems to be moving on.