DON’T let daylight in on magic, the old adage goes.

But when the magic has worn off, then why not? Thus between the writing of this column and its publication, Morgan McSweeney will actually appear in public and, perhaps, explain himself.

For the labour movement the moment is perhaps nearly 10 years overdue. It is hard to find anyone who has done more damage to its cause than McSweeney, the inspirer and, until a few months ago, organiser of the Potemkin premiership of Keir Starmer.

McSweeney’s normal address is the shadows, behind-the-scenes lane. He retreated there to lick his wounds after the candidate he managed in the 2015 labour leadership election, Liz Kendall, came last, polling a handsome 4.5 per cent of the vote.

He drew the obvious conclusion — his cause could only prosper on its chosen battlefield through deceit and deception. Over the years since this has progressed from being a strategy to a morbid reflex.

The tale has now been oft-told, and best in Paul Holden’s outstanding work, The Fraud. His cunning plan, like many before, worked until it didn’t.

It saw his tailors’ dummy of a candidate succeed Jeremy Corbyn on a prospectus which would breach the law on truth-in-advertising in any other line of business.

It allowed McSweeney and his motley crew of sociopaths to smash the left in the Labour Party, not by winning any arguments but through brute force.

It actually lost Labour votes, as compared to “Labour’s worst result since the war” (copyright every palsied pundit who can’t read statistics) but still contrived an election victory thanks to divided opponents and a monumentally unpopular Tory government.

The relationship of dependency which McSweeney, marinated in factional intrigue, established over the hapless Starmer even allowed him to prevail in the power struggle which immediately engulfed Downing Street upon the arrival of its new tenants.

Former top civil servant Sue Gray was dismissed as chief of staff, to be replaced by McSweeney, who she had endeavoured to marginalise.

Did McSweeney actually believe he had the skillset to manage the whole apparatus of government, and give the Labour administration an otherwise absent sense of purpose and direction?

If he did indeed so misestimate his own skillset it was an error so hubristic as to practically challenge nemesis to get its skates on and show up.

One calamity after another — axing winter fuel benefits, cutting welfare for the disabled, suspending MPs for opposing the two-child benefit cap, Starmer’s “island of strangers” turn to Powellism, the hard-line backing for the genocidal Israeli state — all can be attributed to his “strategic genius” (again, copyright the entire media consensus).

This strategy was based on the belief that Labour could only lose votes to its right, and that therefore the attitudes of Reform and Tory voters must be accommodated in government policy.

It has proved a ripe example of fitting strategy around personal preference, rather than what analysis or just sense would indicate.

And what McSweeney wanted was to trash the left in rain and shine, in season and out. That, and nothing else, is why he is in politics to begin with. Opposing socialism is not a by-product of an otherwise substantive agenda, it is starter, main and dessert, with liqueurs afterwards if needs be.

Anything which might suggest that addressing legitimate concerns from the progressive side of politics was necessary had to be expunged from the playbook because — well, because.

Hello from reality. The wheels were coming off this wagon even before the most damaging misjudgement of all — politically at least.

We are speaking, of course, about the immediate matter to hand in McSweeney’s testimony to the Commons foreign affairs committee — the appointment of his mentor Peter Mandelson as ambassador to the United States.

That decision has already cost McSweeney, as well as a gallimaufry of senior mandarins, spin doctors and the like, their posts.

Its ramifications have sucked whatever life there was out of Starmer’s regime. It has exposed the hollowness of the man as well as the poison of his politics.

Lord, we knew not what we did, they all, from the Prime Minister downwards, intone.

But McSweeney knew. He knew that Mandelson had twice been fired from government after imbroglios involving dealings with the very rich.

That Mandelson had further maintained an intimate friendship with the financier Jeffrey Epstein long after his crimes were so much in the public domain that he was doing time for them.

That in the pursuit of further riches, Mandelson had enlisted in the service of various Russian and Chinese companies.

In fact, everyone knew all of that and more about the New Labour potentate, whose sinister shadow has spread over British politics for 40 years. But McSweeney knew something else too.

That Mandelson was his mentor, the sorcerer to his apprentice.  That factional alignment, and nothing else, accounts for sending this louche and serpentine operator to the most prestigious diplomatic post in Downing Street’s gift.

Some may wisecrack — so exactly what is it about a sleazebag with an intense affinity for the filthy rich that makes you think he might get on with Donald Trump?

That leads us up the wrong path. This was not about appeasing the lurid and idiotic US president. Britain already had a career diplomat in post who had established strong ties to the Maga crowd.

There is perhaps a sub-plot in Mandelson’s insatiable desire for the public spotlight and the not-unfamiliar problem of a man who simply cannot accept that his time strutting the stage is done.

Doing relatively unobtrusive good works in the manner of, say, Gordon Brown, was never Mandelson’s way. He wanted the gig and his blood-brother, bonded over opposition to Jeremy Corbyn and his supporters, determined to give him whatever he wanted.

Starmer did no more than sign whatever his chief of staff put in front of him, up until the moment when it dawned on the Prime Minister that the next piece of paper would be the announcement of his own resignation.

That was the moment when nemesis finally knocked at McSweeney’s door. Its work, of course, was not done.

First, it made McSweeney look ridiculous, sending a mysterious street-thief to nick the most sensitive mobile phone in the realm in an episode so traumatic it left the victim unable to tell the police whether he was in Pimlico or Stepney at the time.

Then, having seen off the organ grinder, it turned its attention to the hapless monkey, by now dancing to the old tunes from memory alone.

Thus, humiliation by slices. In many scandals it all comes down to “who knew what when?”

Not in this ever-extending Mandelson epic of chaos and corruption. Here it seems that no-one ever knew anything at all.

The now-departed head of the Foreign Office did not actually see the vetting document which marked Mandelson as a fail.

He also refused to pass it on to the Cabinet Secretary, we have learned. It is unclear whether the Prime Minister has seen it either.  Its contents are argued about intensely by people who relying on disputed verbal summaries.

What procedure was supposed to be followed in nominating, approving and vetting the ambassador to Washington? No-one seems to know that either.

The fallen Olly Robbins followed procedure, he insists. So did I, avers Starmer. Alas, it does not seem to have been the same procedure.

One long-gone cabinet secretary recommended one thing. His successor advised another, before he too was sent on his way. The incumbent appears to have no definitive opinion — wisely, it would seem.

Robbins, Starmer intones, made a “serious misjudgement” and has had to pay with his job. I, too, have made a serious misjudgement, the premier adds, and yet…

Every Labour candidate in next week’s elections is disposable collateral for this politician who unites in his persona the excitement of Rishi Sunak with the probity of Boris Johnson.

Those candidates deserve little sympathy, however. The Labour Party has tolerated Starmer and the whole crooked Labour Together gang — now undergoing an emergency rebrand — for far too long.

One “Labour insider” told the Politico website: “Lots of people think Keir Starmer is a good man who is out of his depth. Wrong. He’s an asshole who’s out of his depth.”

That penny has dropped everywhere except in the Parliamentary Labour Party. That may not be surprising. The PLP was picked by the same hand that picked Keir Starmer himself, the hand of Morgan McSweeney.

But the Labour Party is learning that with McSweeney, as with Mandelson, it is much easier to enter their embrace than extricate oneself. It will be paying for decades for their squalid and self-serving factionalism.