As Spud puts it in the Trainspotting sequel: first there is an opportunity, and then there is a betrayal. Whoever leaked Labour focus group polling on the shadow cabinet’s rising stars to the Sunday Times – allegedly the party’s ousted elections chief, Jon Trickett – intended to capitalise fully on Jeremy Corbyn’s embarrassment last week over Brexit, the undermining of his authority by frontbench rebels and his woeful position in the latest opinion polls.

It is no exaggeration to say that the Tories cannot quite believe what is happening to the opposition. In the words of one of the prime minister’s closest allies: “I sometimes rub my eyes and think I’m dreaming.” The emotions that Labour’s collapse triggers in No 10 are more nuanced than you might think. Every government needs an opposition that it is at least minimally viable against which to define itself and with which to scare the electorate.

That said, the humiliation of Corbyn over article 50 has encouraged those Tories who believe in a fast and furious Brexit, exploiting Labour disarray to drive a deal through parliament well before the general election. Theresa May will not get all she wants from Brussels. But the voters will be left in no doubt that it is her deal, and nothing to do with the has-been party opposite.

On which subject, the most striking feature of the leaked testing by Labour’s pollster, BMG Research, was its focus. Under particular scrutiny were two prominent figures on the left of the party: Rebecca Long-Bailey, the shadow business secretary, and Angela Rayner, the shadow education secretary. For what it’s worth, the focus group in question favoured the former over the latter.

Let us not get carried away. One qualitative testing session in Manchester is not going to make or destroy a political career. Much more interesting was the apparent purpose of the exercise: to identify a potential successor to Corbyn who would appeal to the public but keep the party on the same ideological trajectory. Replace the captain, in other words, but maintain course towards the iceberg.

Corbyn’s champions always blame a supposed “Blairite” fifth column for his travails. But it is the left of the party itself that is now plotting against him most systematically. In December, Ken Livingstone told the BBC’s Sunday Politics: “If it’s as bad as this in a year’s time, we would all be worried.” In the Mirror last month, Len McCluskey, general secretary of Unite, asked pointedly: “What happens if we get to 2019 and opinion polls are still awful?”

Now it emerges that Labour’s left is road-testing its prospective contenders for the next leadership race. The necessary corollary to this is the so-called McDonnell amendment, the proposed change to the rules governing who can stand in such a contest (a plan that takes its inspiration from the failure – twice – of John McDonnell, now the shadow chancellor, to secure sufficient nominations).

At present chapter 4, clause II of the Labour rulebook requires each candidate to be supported by at least 15% of MPs and MEPs (37 names, if the contest were launched today). The amendment, if carried by conference, would lower the threshold dramatically to 5% – a mere 12 parliamentary cheerleaders.

In 2015, a number of MPs (including Margaret Beckett, Frank Field and Sadiq Khan) munificently nominated Corbyn to ensure that all sections of the party were represented in the contest – never dreaming that he would triumph. It is a safe bet that there will be no such magnanimous vote-lending next time. So the left – given its relative weakness in the parliamentary party – requires the amendment to be carried if it is to have a candidate in the race to succeed Corbyn.

As reliably bad as the left is at winning general elections, it is extremely good at organisation, internal party strategy and building factions. Its adherents are the ones who show up at meetings and stay to the end, who stand for local party office and find their natural habitat in committees and caucuses. It is lazy to describe this as “infiltration”. Such behaviour reflects application, conviction and stamina.

What should alarm all of those who believe in the desirability of an electable Labour party is how badly prepared, by comparison, are its leading moderates. It would be the work of five minutes to name a plausible centre-ground Labour cabinet: Chuka Ummuna, Stella Creasy, Rachel Reeves, Yvette Cooper, Keir Starmer … the list goes on.

There is no shortage of talent among the party’s centrists. But where is the coordination, the focus, the rage for victory? At a moment of maximum danger – and opportunity – they are barely ready for a game of rock, paper, scissors, let alone a battle to the ideological death.

Yes, there are meetings with skinny lattes to discuss tactics over and the precise meaning of “soft Brexit”. But the ferocity and camaraderie of the mid-90s – the sheer juggernaut energy – is entirely lacking. “I feel like an army of one,” I was told recently by a talented senior figure who should be already be surrounded by a praetorian guard of ruthless supporters preparing a leadership bid.

Equally, the party’s moderates need to move on definitively and unambiguously from the Blair-Brown era. This is not to say that they should disavow New Labour’s determination to build electoral coalitions, to reconcile social justice and capitalism and to identify the party with law and order and aspiration, as well as with the NHS and equality.

Quite the opposite: the mission for Labour’s centrists is to graft these resilient objectives on to a landscape that has changed fundamentally since 1997. Twenty years ago, Labour’s task was to prove itself at ease with globalisation. In 2017, it must show that it understands the pathologies of that system – pathologies that are driving rightwing populism around the world – without allowing the party to retreat into a la-la land of wishful thinking and confirmation bias.

Two critical byelections approach. So do the local and mayoral elections in May. The Labour leader – more resilient than he is often reckoned – may yet hang on until 2020. But the countdown to his departure has begun, and the party’s moderates are not ready. Now might be a good moment to fix that.