"Like a flock of starlings changing direction".

That's how one former Labour official recently described the sudden movement of Labour voters to the SNP in former Labour heartlands. Yesterday's Lord Ashcroft opinion polls confirmed that anecdote in dramatic fashion, revealing none of Labour's safe seats can any longer be described as safe.

Why did this Tory Lord's opinion poll have such a dramatic impact? Haven't polls been showing for the last four months that the SNP has a 20 per cent lead over Labour in voting intentions for Westminster?

Well, the shock - and it was seismic - is partly because there had been weeks of speculation that these unique constituency polls, with samples of 1,000 per seat, would reveal the SNP's humungous lead in national polls has not been be reflected on the ground. That long-standing Labour voting habits linger on.

But it seems they haven't. Across the range of Labour's heartland seats - Glasgow, Dundee, Renfrewshire...- the SNP has built an even greater lead of 25 per cent. Of the 16 safe seats polled, only Glasgow North East MP Willie Bain is left standing on these numbers. Gone would be: Shadow Foreign Secretary Douglas Alexander, Shadow Scottish Secretary, Margaret Curran, Anas Sarwar, Gregg McLymont, Tom Harris, Ian Davidson....

What seems to be happening is that voting intentions for Holyrood are being translated into voting intentions for Westminster. The fear of "letting the Tories back in" has subsided for the time being as attention has focussed almost exclusively on Scottish politics in the wake of the independence referendum.

Labour's hopes that its dominance of Scotland's working class would endure at least in Westminster voting are cruelly dashed. Three months from the general election Scotland is looking like a lost cause. And it isn't just Scotland that's down the swannee; if these polls are right, Labour may already have lost the next general election.

Certainly, the days are over when Labour votes were weighed rather than counted in Scotland. When a monkey in a red rosette could reasonably expect to be returned to Westminster if it chose the right seat. Labour now has to fight for every single vote. And it faces an SNP with nearly a 100,000 members, a formidable treasure chest and the most professional electoral machine it has ever had.

Moreover, it seems that nationalism is replacing class as the driving force in Scottish politics, at least in former Labour areas. The irony is that the independence cause is actually doing rather less well in the safer SNP seats in the North East. However, Lord Ashcroft's polls suggest Alex Salmond will nevertheless win Gordon and that Danny Alexander, the Liberal Democrat Treasury Secretary, is doomed in Inverness.

The Ashcroft map of Scottish politics has turned almost uniformly SNP-yellow, raising fears Scotland could make a seamless transition from being a Labour one-party state to an SNP one. Some are hoping this prospect will make ex-Labour voters pause and reflect. But it may make Scottish voters even more determined to vote SNP on the grounds that nothing succeeds like success. They want to make a statement.

The independence referendum, it seems, was a transformative event that drew a line under a century of working class industrial politics in Scotland. It marks the decline not only of the Labour Party in Scotland, which has fewer members now than Nicola Sturgeon can command for one event at the Glasgow Hydro, but also the wider Labour movement.

It was the trades unions above all that created and sustained the Labour Party in the 20th century. But industrial Scotland is now a distant memory for all but the over-65s (the only group according to Lord Ashcroft who still support Labour). The unions have been hollowed out, council housing estates have been broken up, local authorities where Labour used to dominate have been reformed by proportional voting.

The social structure that sustained the Scottish Labour Party has been transformed and the new generation growing up in post-industrial Scotland finds the nationalist message more compelling than the old Labour religion. Concepts like workers solidarity, proletarian internationalism, or Gordon Brown's "caring-sharing" socialist unionism mean very little to them. Nationalism seems brighter, more immediate and altogether more emotionally engaging than the dusty dialectics of old-style marxism, still less the managerial politics of the Labour Party.

The SNP has skilfully absorbed much of the social democratic content of the old Labour Party - in the way the Medieval Christian Church absorbed many of the old pagan festivals - and turned it to its own advantage. The idea of creating "a better nation" seems more plausible a project to younger Scottish voters than the task of recreating the old social democratic UK that won the loyalty of their grandfathers after the Second World War.

The dysfunctional politics of Scottish Labour has accelerated its decline. Even before the hapless Johann Lamont admitted after the referendum that Labour was a "branch office" of the UK party, Labour had marked its own card. The succession of incomprehensible scandals during the Holyrood years and the resignations of two party leaders undermined its credibility.

The tongue-tied ministers, the corrupt councillors, the Subway-standard leaders tarnished Labour's appeal. As did the ideological contortions it underwent in the effort to reconcile its own social democratic instincts with the wave of "modernising" market reforms emanating from New Labour under Tony Blair.

The final nail was the participation with the Tories in Better Together. It wasn't just the sight of Labour politicians sharing photo ops on Calton Hill with Tory leader Ruth Davidson that alienated voters. It was the way Labour eagerly fronted a campaign that was defined by the interests of the banks, big business and establishment figures like the Tory Chancellor, George Osborne.

It was Ed Balls gleefully endorsing Osborne's Declaration on the Pound; Ed Miliband saying he would put in Labour's very election manifesto that Scotland would "never" be permitted to use a currency that many naively believed was the joint property.

Labour get this now. Jim Murphy gets it more than most, which is why - despite his own Blairite background - he has tried to reinvent Labour as a kind of small 'n' nationalist party. But the danger of being more Nat than the Nats is that it can make the SNP seem like the Real Thing. Not so much triangulation as strangulation.

Of course, it's not over till it's over. Like the starlings, the flock could suddenly break back to Labour during an election campaign dominated by UK politics. First Minister Nicola Sturgeon certainly isn't counting her starlings and warns of the danger of the SNP being "squeezed out" by a UK media which concentrates overwhelmingly on the Westminster parties and their differences.

But a last-minute reprieve for Labour, while possible, is looking less and less likely. Scotland has changed. It changed in the crucible of a referendum campaign which, for the first time, gave meaning to politics for a new generation of working class voters who have given up on Westminster politics. Holyrood has shown the way, and they like it. Scots are doing it for themselves.