For more than two years, ever since Jeremy Corbyn won the first of his two overwhelming Labour leadership victories, there has been a tendency, widely shared across the British political spectrum, to see “Corbynism” as one thing, a unified leftwing project. Yet Labour’s current internal debacle over the selection of its next general secretary, due for completion next week at a meeting of the party’s national executive committee, should explode that laziness.

True, it now seems certain that Jennie Formby, the candidate of the Unite union, will win an easy victory in the selection race as Labour’s top official, as she will be unchallenged by any of the major alternatives. True, this will be seen, rightly, as further consolidating Corbyn’s hold over the Labour party machine after the resignation of the previous general secretary, Iain McNicol of the GMB union, a Corbyn opponent.

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Yet Formby’s victory has not been smooth. It took heavy arm-twisting from Corbyn himself to finally persuade the Momentum founder and veteran grassroots left activist Jon Lansman to abandon his own candidacy last weekend. And it was only this week that the GMB, historically the most centrist of Labour’s big trade union affiliates, decided against nominating its own Lisa Johnson for the job. It is not clear yet what price Lansman, in particular, extracted for his agreement to withdraw.

Within little more than three weeks, the general secretaryship issue has highlighted major faultlines within Corbynism. Some of these faultlines – trade unions versus individual members, radical labourism against socialism, and Unite versus the GMB – are 21st-century iterations of divisions that have existed ever since Labour was founded in 1900.

It has often been noted that Corbyn’s support stretches across a wide demographic spectrum, from grizzled, grey-haired, baby-boomer leftists to student and cyber-age political neophytes, and from trade union leaders to newspaper columnists. The temptation, among both Corbyn supporters and opponents, has been to present this as a sweeping victory for a unified left movement, with a shared worldview and programme, riding the wave of popular anger against Iraq, New Labour and austerity.

But that is really only the starting point. The deeper truth is far more complicated and fluid. The general secretaryship race has exposed to public view something that in reality has been there all along – real and potentially fundamental divisions of political thinking and strategy that matter. They could become acute and difficult to manage if Corbyn ever leads Labour into government.

There are many ways of illustrating these differences. One was very starkly displayed this week over the Labour leader’s reaction to the Sergei and Yulia Skripal poisonings. Consistently anti-western on foreign policy issues, Corbyn has been reluctant to support a pushback against Russia. A succession of Labour backbenchers took a much more forceful line. So far, so familiar, you may feel, thinking back to the Syria debate of 2015. Yet although some of Corbyn’s allies are lifelong adherents of the “my enemy’s enemy is my friend” approach to foreign policy, plenty of others are not. Some of the most uncompromising anti-Russian comments by Labour on social media this week have come from Corbyn backers such as Paul Mason.

Until now, Corbynism has been able to ignore many of the tensions between its centralising and its grassroots tendencies

Brexit is another example. Corbyn is, at best, tactically ambivalent about Brexit, as he made clear most recently in his speech to the Scottish Labour conference last weekend. Again, a few Corbynites (mainly the older ones) are, like the Labour leader himself, anti-EU in the Bennite tradition. Yet the majority, like the majority of Labour voters, are committed remainers.

Arguably the most important re-emerging divide, however, is about control versus democracy. The left has often been split over democracy. The divide goes back at least to the arguments between Marx and Bakunin that wrecked the First International in the 1870s, and even to the split between the Jacobins and Girondins in revolutionary France. Now Corbynism is divided over the issue too.

Until now, Corbynism has been able to ignore many of the tensions between its centralising and its grassroots tendencies for the simple reason that Corbyn himself is so clearly the party’s democratic choice. Yet the top-down approach of some on the left, especially from the Stalinist and the union traditions, dedicated to controlling the levers of power in the party and the state, is always going to find itself in some kind of conflict with the left’s bottom-up traditions of local activism, shopfloor democracy and, in the cyber age, of interlinked social media communities.

The general secretaryship argument has blown that issue wide open once again. The most striking expression of it was the Momentum activist and NEC member Christine Shawcroft’s Facebook comment last week (later deleted): “Nothing would induce me to support a candidate from a major trade union.” It was time, she added, “to support disaffiliation of the unions from the Labour party”.

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It is hard to think of a greater traditionalist Labour heresy than this statement. But Shawcroft is not alone. And her comment highlights a gulf between, at its extremes, those Corbynites who essentially want to reproletarianise the party, on pre-1980s lines, and those who want the 21st-century Labour party to move beyond the remnants of the industrial proletariat, and even leave it behind, in favour of what some see as a movement of post-national, urban, identity-driven and networked youth.

Corbynism, in short, is not one thing but many things. Many Corbynites have feet in more than one camp. And, while history never quite repeats itself, Labour has been here before. In the 1980s, the Bennite surge embraced old top-down Marxist sectarians as well as post-1968 radical social movements. Yet Bennism failed because it still wasn’t broad-based enough, and because its different components turned their backs on too many Labour voters. The result was the revival of a more voter-aware “soft left”, which eventually reunited the party.

That does not mean Corbynism is certain to fail too, let alone in the same way. For one thing, Corbyn controls the party, which Benn never did. For another, the Conservatives are far less confident today than in the 1980s. But the signs of what went wrong for the radical left in the past are beginning to reappear. Whether there is a new version of the 1980s soft left that is able to take advantage is, however, another question.

• Martin Kettle is a Guardian columnist