It is no longer a question of if or why Ed Miliband will cease to lead Labour, but when.

Despite the desperate denials and calls for unity, his party has begun, finally, to catch up with public opinion. With their usual brilliance, spooked Labour MPs have come to understanding too late.

They are stuck with Mr Miliband. Those urging a general election campaign based on the party rather than its nullity of a leader know that much, at least. "Defend the marginals!" might not be much of a battle-cry, but it is better than pinning your prospects on a personality who manages, astonishingly, to be less popular today than Nick Clegg.

Labour cannot dump Mr Miliband six months before polling day, whatever the slim reported hopes of depressed MPs from the English north-west. What would that say about the party? More importantly, what message would it convey to voters about the arguments the Opposition has struggled to convey since 2010?

To have any point at all a new leadership would have to chart a new course. That would mean the disavowal of past errors. That would mean admitting that the Eds, Miliband and Balls, embodied a huge misunderstanding of the public and the purpose, if any, of Labour. One way or another, it would mean a renunciation of everything the electorate have been asked to believe hitherto.

A minority, barely a third, have any belief in the party. Those prepared to suspend disbelief for the sake of the leader of the Opposition are fewer, vastly fewer, still. The excuse that the Tories and LibDems are also suffering amid so-called "anti-politics" is no excuse at all. Mr Miliband had one job. It fell to him to carry the hopes of millions victimised by the coalition's austerity programme and he blew it, time and again.

So much has been evident for years. The idea that David Cameron and George Osborne could ever enjoy more trust as economic managers than Mr Miliband and Mr Balls ought to have been comical from the moment the Chancellor delivered his emergency budget in 2010. Since then, the coalition's "long-term plan" has been an improvised excuse, revised quarterly, while the UK's problems of debt and deficit have persisted.

For Labour, it should have been fish in a barrel time. The Tory claim that the Gordon Brown government caused the financial crash should have been swept aside easily enough. The idea that Mr Miliband and Mr Balls, veterans of that administration, could be held accountable for the continuing failures of the coalition, year upon year, should have been laughed out of court.

Instead, Labour fell for the "deficit deniers" charge. It accepted the Tory narrative and Mr Osborne's spending limits. It accepted, with every appearance of eagerness, the coalition's vindictive assault on social security and its pandering over immigration. Most Labour MPs went along with all of it, betting on a foolish "35 per cent strategy" for government and the claim that Messrs Miliband and Balls would do something - but nothing too drastic - about the scourge of low pay.

It's old news. Those who had no desire to see this Miliband brother as their leader will no doubt continue to relocate their cherished centre-ground ever closer to the Tory right, as though becoming your opponent is the only remaining point in British politics. Such people favour tactics - you wouldn't call it a strategy - of explicit exclusion. For them, only certain voters matter. By no coincidence, Mr Cameron has made the same sort of bet. It no longer looks like a clever bet.

Labour types demanding calm from a panicking party offer a couple of daydreams. In one, it is imagined that an election campaign will be survivable even with a leader who has managed to lose the support of many who voted for Mr Brown. These optimists appear to be serious. They seem to think voters will pick "Team Labour" and forget to ask about the person offering himself as Prime Minister, especially - and this is the truly daft part - in marginal seats.

The second piece of whimsy sees Labour crawling over the finishing line in May as the largest party, but forced, "in the national interest", to seek coalition with surviving LibDems. That might yet happen. Mr Miliband could no doubt talk himself into the stratagem. So how much would be achieved by such desperate measures? How would it address contempt for Westminster and the politics still described as mainstream? How would it renew and advance Labour?

It's all they've got. No one believes for a second that Mr Miliband will sweep to power. In fact, rational Labour people have a suspicion he will be blown into burbling bits in a serious election campaign when the Tories play dirty, Ukip presses across the length and breadth of England, and the SNP advances in Scotland. It's not complicated: people who have voted Labour in the past look at the party leader and say, in growing numbers, "Don't be ridiculous".

The idea that Alan Johnson could be recruited as a caretaker by the year's end is fanciful. First, he has denied any interest in becoming a contender, and done so repeatedly. Secondly, if - for you never know - he has a sudden change of heart, it would make far better sense to see the light next May than now. Who wants to be a stop-gap? More to the point, who votes for a stop-gap?

Andy Burnham, the shadow Health Minister, has his fans. As shadow Home Secretary, Yvette Cooper is mentioned with inevitable regularity. No one else in Labour's ranks, least of all David Miliband, that prince across the water, is a serious proposition. Yet since Ms Cooper is married to Mr Balls, any hopes on her part - vehemently denied, of course - have practical consequences. Mr Burnham's difficulty is less complicated: who wants to be the disloyal figure who risks "losing Labour the election"?

None of these charades are close to the heart of the matter. Labour has endured a crisis of identity ever since Tony Blair and Mr Brown sought to make a personal design project of a party. So much is self-evident in Scotland. Amid the complaints over an anti-politics insurgency, the real question is forgotten: when - and why - did Labour cease to be an insurgent force?

When capitalism trembled on the brink of collapse, the party's leaders picked their side. Much good it did them or Labour. For a majority of Scots these days they are, unambiguously, the voices of a rejected establishment.

They did their best to conceal the fact during our referendum, of course. What was the deal back then? Vote No, vote Labour, sweep the Tories out of Westminster, and all will be well? Some people wanted to believe it. Some voters even managed to believe that still another heave would happen thanks to the willing shoulders of Mr Miliband.

Most in his own party did not believe a word of it in September; precious few believe it now. Anything they claim, as the weeks tick by and Mr Miliband mistakes truth for "nonsense", should be judged against the fact.