Speaking about his spectacular weight loss not long ago, Tom Watson said he felt “like a cloud has been lifted from my brain. My memory is a lot better and I’m more compassionate. Maybe I’m even too chilled out for politics now.”

Jeremy Corbyn doesn’t have too much to smile about right now, but this quote might tease a wry grin. If Labour’s deputy leader has more compassion, he isn’t wasting a smidgeon on his nominal boss. As for being too chilled out for politics, maybe and also maybe not.

With the ides of March imminent, Corbyn won’t need a Watson-class memory to remember the words Shakespeare put into Julius Caesar’s mouth. “Let me have men about me that are fat, sleek-headed men and such as sleep a-nights. Yond Cassius has a lean and hungry look … Such men are dangerous.”

Then again, you don’t need Watson’s faultless memory to recall that his girth seems irrelevant to his willingness to assassinate Labour’s emperors.

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Back in the chunkier days of 2006, after the calorific Indian meal known as “the curry house plot”, he launched the knife and fork attack that forced Tony Blair to name his departure date.

Some assumed he did that under instruction from his liege lord Gordon Brown, to whom he’d just paid a house call in Kirkcaldy. Watson brushed the circumstantial evidence aside, insisting he underwent the 800-mile round trip to deliver a cuddly toy for Brown’s new baby in person.

And behold him now, a dozen years later and almost as many stones lighter, up to his old tricks.

This coup attempt might be subtler than his last, and the beneficiary in his mind might even be himself. But even in these eccentric political times, this is a highly peculiar brand of coup.

With Corbyn still in command of Labour’s levers of power and the membership’s support, Watson appears to be creating, or trying to create, a shadow party within the shadow government.

His reason, or his excuse, is antisemitism. The decisive hand behind the suspension of Chris Williamson, and openly usurping the general secretary’s disciplinary role, Watson described Luciana Berger’s departure as “the worst day of shame in Labour’s history”.

You can share his distress over Berger’s maltreatment (as I do; she’s a fine and brave MP who was treated abysmally) without necessarily regarding her defection as Labour’s all-time nadir.

For that, you might cite the day in 1968 when Jim Callaghan, then home secretary, removed the right of Kenyan Asians with British passports to settle in Britain with a piece of legislation likened by the revered anti-racism campaigner Enoch Powell to Nazi race laws.

Rather more would give the dishonour to the night in 2003 when the majority of Labour MPs voted to invade Iraq. However atrocious the terrorising of one innocent Jewish MP, approving the slaughter of untold hundreds of thousands of Muslim innocents was possibly worse.

Watson supported that, though unlike many of his colleagues he publicly regrets it. In January, he said it still keeps him awake.

Heaven loveth the sinner that repenteth, but here on earth we tend to be more sceptical. After supporting that war, as any fat, sleek-headed henchperson of Brown would, he didn’t mention the insomnia for a very long time.

Now that he has that lean and hungry look, Watson ostentatiously claims that he no longer “sleeps a-nights” from guilt.

Even his humility has its limits. Despite admitting to that monumental misjudgment, he has no respect for the Corbyn who called it right on Iraq. Nor does he have the career-risking bravery of Berger.

Instead, he is using his position to manufacture a semi-official opposition within the official opposition. His musings about an alternative PLP catering to (and even perhaps whipping) the 100-plus MPs at the more social democratic end of the party spectrum might be cutely opportunistic.

You’d have to be terminally nose blind not to sniff the mood shift. Corbyn’s mishandling of the antisemitism disaster and transparent Brexit game-playing have combined to alienate many natural supporters.

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His leadership ratings are at a historic low, and his grip on the parliamentary party weaker than ever. But however enfeebled Corbyn is, however plainly he is blowing Labour’s shot at removing this abomination of a government, he was democratically elected under the same rules by which Watson became deputy leader.

If he could remove Corbyn according to those rules, he would. Instead, like the hapless leaders of the abortive coup against Gorbachev, he is trying to place Corbyn under house arrest in his dacha.

However this power struggle pans out, it will weaken Corbyn without killing him and damage the Labour movement.

The doctrinal schism ailing Labour can’t be cured like the type 2 diabetes Watson says he no longer suffers since losing the weight. Rather, it is the equivalent of Theresa May’s type 1. It has to be carefully, patiently managed if it isn’t to be fatal.

What Watson is doing is careless and impatient, and can only exacerbate it. If he tells himself this is his duty as a Labour loyalist, he doubtless told himself the same when he voted to unleash hell on Iraqis.