Theresa May will not, we are not told, be triggering an early general election this year. Good news for political journalists and the Great 2016-17 Sleep Deficit – although the proposed triggering of Article 50 in nine days’ time will keep everyone occupied. Less good news for the Labour Party.

You’d be forgiven for thinking Labour HQ will be thanking their lucky stars to have avoided a bloodbath on 2 May. ICM polling, out today, gives the Tories a 19 per cent poll lead over Labour; projections from the Electoral Calculus polling organisation, borne out by the by-election results in Stoke and Copeland, show May increasing her working majority from 17 to 140.

MPs from both ends of the Labour Party are shivering. Last week, one of Corbyn’s parliamentary critics told me he sits on a constituency majority of several thousand – “but the Tories will take it, easy, if there’s an election any time soon”. Less predictably, in an interview with the i newspaper this week, John McDonnell admitted that it may take Jeremy Corbyn “18 months, 24 months” to turn around his poor polling performance.

So why won’t Theresa May deliver the killer blow? Why not put Labour out of its misery?

If you listened to Monday’s Today programme, you’re halfway to the answer. In an extraordinary interview, deputy leader Tom Watson, a man who normally likes to let other people deliver his punches, accused the Unite union and the Momentum activist group of “a hard-left plan to control the Labour Party”. The Labour Party is at war – “this as a battle for the future existence of the Labour Party”, per Watson.

Momentum looks like it is orchestrating a 'takeover from the hard left' says Tom Watson

Watson was talking about a tape of a Momentum meeting, leaked to Sunday’s Observer, in which senior Momentum leader Jon Lansman appeared to outline a strategy for ensuring that Unite and Momentum activists ensure the election of their own candidate should Jeremy Corbyn stand down. The real scandal? The Unite movement – which currently channels funds to the Labour Party – appears to be considering redirecting that money to Momentum itself. That eventuality relies on the re-election of Len McClusky as general secretary of the Unite union, another organisation which Momentum activists are accused of infiltrating.

All this might sound like a minor coughing fit in Labour’s inevitable death throes. But the issue of who gets union funding is key to the existential identity of the Labour Party. Labour began life as the political arm of the trade union movement when the first great battles for workers’ rights were being fought – its continued claims to represent anything like the working man remain predicated on that link to the union movement. For Unite to shift its funding from the Labour Party proper to an activist faction within it – and not all Momentum activists are even members of the Labour Party – would represent not just a crippling loss of funds, but an end page on its founding myth.

Just as dramatic would be any break by the Labour Party itself from its long-time union backers. It’s been tried before. In 1981, when the centrist “Gang of Four” broke off from the Labour Party to form the Social Democratic Party, they did so after a one-day party conference at which members voted to hand the largest say in any future leadership election to the trade union bloc. The resulting “Limehouse Declaration”, currently the subject of a new play at the Donmar Warehouse, has been much mooted as a model for moderate Labour MPs considering breaking from the Labour Party completely.

Except, of course, that the SDP failed. One reason they failed was Roy Jenkins, David Owen, Shirley Williams and Bill Rodgers had created a party without history. They had grassroots – thousands joined up across the country to campaign for this new left-of-centre movement. And leftist centrism clearly has appeal to voters: Tony Blair was effectively the final triumph of the SDP in government.

But even if Margaret Thatcher hadn’t jacked up her popularity with the Falklands War; even if the SDP had given themselves more time before they gave up and merged with the Liberals (it takes 10 to 15 years of constituency organisation to take a seat): the party didn’t have a heroic story to sell to former Labour voters. In Steve Waters’ new play, the characters of Williams, Jenkins and Rodgers tell each other stories of how the Labour unions won strikes, fed workers’ widows and, in the case of Jenkins, saved his father from an unjust prison sentence. “We will be hated,” says Rodgers. “And they'll be f***ing right.” For many long-serving activists, to discard a Labour rosette to side with those who transported the Tolpuddle Martyrs to Australia was unthinkable.

As it is, Labour moderates lack the personnel to set up an appealing alternative to contemporary Labour. Many MPs are simply giving up. Tristram Hunt is not the only high-profile Labour figure to take an escape parachute in the form of a prestigious establishment job: Baronness Royall, the well-regarded former Brownite who steered a thoughtful path between all politically motivated factions in her report on anti-Semitism in Labour university clubs, recently announced that she will become head of Somerville College at Oxford University (on hearing Momentum activist Christine Shawcroft defend the latest moves this morning, Royall made her feelings known.)

Head-to-head: Theresa May v Nicola Sturgeon

What does all this have to do with when Theresa May should call a general election? It’s certainly proof that the Labour Party is in trouble now. But it also demonstrates that Labour’s internal divides are likely to get a lot worse before they get better.

The most revealing thing about Jon Lansman’s leaked remarks to fellow Momentum activists is that the Corbynistas-in-chief have started planning for life after Jeremy Corbyn. This was suggested when “annointed heir” Rebecca Long-Bailey was promoted in last month’s reshuffle. That means another leadership election, and before the leadership election, the necessary implementation of Lansman’s plan for complete Unite/Momentum control of the process. For those, like Theresa May, who believe that a credible electoral challenge can only come from Labour’s centrists, the prospect of running against a Len McClusky candidate in 2020 is too good to resist.