One of the many things that have changed about the Labour party over the past two years is that some of its leadership cadre now cite Marxist thinkers with approval. John McDonnell, the shadow chancellor, likes to quote Antonio Gramsci. The Italian communist theorist is probably best known for his advocacy of “the long march through the institutions” – the aim being to conquer them one by one for the revolutionary cause.

This week the Corbynite left’s long march through the institutions of the Labour party will achieve another victory when they cement control of the national executive committee. Jon Lansman, a veteran of the Bennite wars of the 1980s and founder of Momentum, is expected to win one of three new seats on the NEC. The other two extra seats, created to give more representation to the membership, are also forecast to be won by Momentum-recommended candidates.

This, along with other recent changes to the composition of the NEC, will be another significant ratcheting up of the Corbynite grip on the Labour machine. They occupy the commanding heights of the leadership. They have the shadow cabinet, though with the important caveat that some of the Labour frontbench are Corbynite in name only and others not even that. They have the backing or current acquiescence or the current acquiescence of the leadership of most of the trades unions.

Now he will also have a reliable majority on the party’s ruling body. The NEC has the power to propose constitutional changes so control of it can be used to alter the way policy is made and revise the operation of the annual conference. If they want to do so, they can pursue changes to the party’s rules which will make life even more uncomfortable for its non-Corbynites. Never before in the party’s history has the left controlled so much of Labour. Messrs Corbyn and McDonnell, men who spent their previous careers railing against the party’s elite, are now its establishment.

This is a transformation of Labour as dramatic as anything that happened under Tony Blair in the 1990s. Arguably, it is much more significant. During his time as leader, Mr Blair did not put in much organisational effort to ensure that his ideological shift was a permanent one. New Labourites just assumed – how wrong they turned out to be – that the hard left had lost the arguments so comprehensively that it could not revive. New Labour never attempted to purge rebellious leftwingers, not even when they were as serially mutinous as Mr Corbyn himself. The Corbynites have absorbed that lesson and want to make Labour’s red shift irreversible.

Yet absolute control still eludes them; the long march through the institutions is not complete. There is one very important power bloc they do not control: the party’s elected representatives. Most Labour councillors are not Corbynites. The party’s most prominent mayors – Andy Burnham in Manchester and Sadiq Khan in London – are not Corbynites. The majority of Labour MPs are not Corbynite.

It is a concern that Labour has not gained a more decisive advantage over the Tories during a fabulously bad period

The Labour leader’s internal critics have largely fallen silent over the past six months, muted by the party’s above-expectations election result and Mr Corbyn’s popularity with the membership. Virtually every Labour MP now recites the mantra: “Jeremy is leader for as long as he wants to be.” That does not mean that they are all truly reconciled to his leadership or what has happened to their party. Most Labour MPs are some variety of social democrat or socialist. They do not cite Marxist theorists with approval. Many Labour MPs think the party could have done better at last year’s election under a leader who aroused less distrust among centrist, working-class and older voters. Others acknowledge that Mr Corbyn performed well, but worry that there will be a cap on the party’s support for so long as he or anyone of his ideological flavour is in charge. Quite a lot of Labour MPs think it is entirely possible that their party will win the next election – and then preside over such an economic disaster that it will ruin Labour’s reputation with the electorate for a generation.

Jon Lansman at a Momentum campaign rally in support of the Labour party leader, Jeremy Corbyn. Photograph: Jake Mather/HMG/Rex/Shutterstock

Since these MPs are only pretending loyalty to the leader, some Corbynites say they ought to be purged and replaced with a parliamentary party that can be relied on to support the cause. A Corbyn government would have a lot of enemies. Mr McDonnell has talked openly about “war-gaming” for a financial crisis. He and Mr Corbyn would no doubt prefer to have Labour MPs who are fully committed to the project rather than a parliamentary party packed with moderate social democrats waiting for it to go wrong. This – along with old-fashioned, sectarian revenge – is an argument for taking on their opponents in the parliamentary Labour party.

There are obstacles in the way of taking this course. Momentum has become an influential force, but it is not so potent as it is sometimes portrayed. The north London borough of Haringey, where an unpopular housing regeneration project has been used as the spearhead to deselect councillors and replace them with Momentum-approved candidates, has generated a lot of headlines. Less attention has been paid to battles where Mr Lansman’s organisation has struggled to make inroads. I offer one example of many available. After recent selections of Labour candidates for this May’s local government elections in Manchester, Momentum will probably end up with eight of a likely 96 Labour councillors “and two of the eight are only pretending to be Momentum”, according to one well-placed observer.

The juiciest prize is control of the parliamentary Labour party. Some Labour MPs are finding that their local parties have been overrun by hard leftists who want to lever them out. The first step in that process is usually the capture of constituency posts such as membership secretary and treasurer. At minimum, this can make life very miserable for an MP out of sympathy with Corbynism. It could also pave the way for deselection. Angela Eagle in Merseyside and Chris Leslie in Nottinghamshire, both of whom have held senior positions on the Labour frontbench, are among those having a torrid time. One Labour MP describes the atmosphere in some constituency parties as “feral”.

The bigger picture suggests that it will not be easy to dislodge Labour MPs in large numbers. Many have been developing survival strategies. “Get organised” is the advice of one Labour MP who has successfully prevented a hard left takeover of her local party. Canny MPs have been careful to build relations with the new members who have surged into Labour since Mr Corbyn became leader. There is a distinction to be made between the ideologists and the idealists. The ambition of an older generation of hard left activists to take out moderate Labour MPs is often not shared by younger members who love Mr Corbyn, but don’t identify with the hard left and aren’t attracted by its ugly factionalism.

The leader and his inner circle also have to consider Labour’s present position in the polls, most of which have the party slightly ahead or neck and neck with the Tories. Though they’d never like to let on publicly that this bothers them, it is a concern to at least some of Mr Corbyn’s team that Labour has not gained a more decisive advantage over the Conservatives during a fabulously bad period for the government. Embarking on divisive attempts to purge some of its own MPs is not a self-evidently smart way to try to broaden Labour’s appeal.

So the assumption among most Labour parliamentarians is that Mr Corbyn will not want to unleash a wave of deselections – at least not yet. “If they are rational, they will be wary of starting a bloodbath,” remarks one MP. Some speculate that Mr Corbyn and Mr McDonnell will think themselves best served by just keeping the threat of deselection dangling over the head of the parliamentary party in order to keep it cowed and mute.

Any Labour MP who is told that he or she will not be allowed to stand for the party at the next election could always carry on sitting in parliament in the meantime, using the platform to say exactly what they think about Corbyn Labour. Or a deselected MP could resign and fight a byelection which they might win if they were popular locally.

The biggest risk to Mr Corbyn of attempting a purge is that it could start a chain of events that lead to the launch of a breakaway party. Chatter about the creation of some kind of new party has subsided since the general election, but it is still rumbling away in the background. Mr Corbyn’s inner circle have long been nervous of the emergence of a British version of a Macron-style force. They do not think it could beat them. They do fear that it might attract support from liberal voters and social democrats in sufficient numbers to deny Labour power the next time that Britain chooses a government.

So the Corbynites’ latest victory in their march through Labour’s institutions presents them with a conqueror’s dilemma. Some may be tempted to try to create a parliamentary Labour party that will be more Corbyn-compliant if he makes it to No 10. The risk of pursuing that course is that it could very well lengthen the odds on him getting there.