The ongoing struggle to be Labour’s general secretary is a sign of just how hegemonic the party’s left has become. This might sound like an odd reading: the contest between Unite’s Jennie Formby and Momentum’s Jon Lansman has shown worrying flashes of acrimony, both on Twitter and in certain anonymous briefings that have dispirited figures on both sides.

To an outside observer, a struggle between Unite and Momentum – both of which have played instrumental roles in the rise of Britain’s radical left – might seem to have a hint of the People’s Front of Judea about it. But it is a mark of the left’s sense of political security that different factions can attempt to assert themselves over one another, because it no longer fears a recapture of the party by the right. Whoever wins, the real question is: can Labour become a members-led democratised mass movement?

Superficially, the contest seems bewildering. If either Formby or Lansman wins, the party machine will be firmly in the hands of the left, and the party’s structures will be further democratised. So why the split? The leader’s office – or Loto as insiders describe it – backs Formby partly because it is keenly aware that the party is too dominated by men.

The general secretary position is an organisational role, and Loto emphasises Formby’s proven record as a union organiser with demonstrably superior management experience to Lansman. She was close to both Jeremy Corbyn and John McDonnell back when the Labour left was near-extinguished politically: McDonnell once tipped Formby as a future Unite general secretary, long before Len McCluskey assumed the role. Her politics are firmly rooted on the left, combatively so. In the first leadership race, Formby enthusiastically saw one of the upsides of a Corbyn leadership as a means to challenge or replace officials in Labour’s Southside headquarters who have loyalty to the party’s right flank. This may explain hostile briefings against her emanating from some quarters.

While Formby is a firm union leftwinger, Jon Lansman ran Tony Benn’s 1981 deputy leadership campaign and is Bennism in human form. Bennism holds party democracy to be sacred, and on a point of principle Lansman believes important positions should be open and contested. Rather than seeking conflict with Unite, above all else, Lansman is standing to open up the contest.

Lansman loyalists speak of an aspiration to build a “broad, pluralist party”, and fear the signals that a closed contest for such a critical role sends out. His lifelong obsession is creating a grassroots-led party, and a democratisation agenda taken to its logical conclusion may well face moments of opposition from both union hierarchies and Loto.

Momentum is an organisation in a hurry, because its leaders worry that Corbyn’s successor may not be rooted in the left, which makes structural changes to the party rather urgent. The question has also been put to me of whether it would be healthy for either the Corbyn project or Unite if the latter became so utterly dominant in the party’s top echelons (a view shared in other affiliated unions).

These are nice problems for the left to have. While some of Formby’s supporters worry that a more rightwing candidate could slip through the middle, a left-dominated National Executive Committee – which will make the appointment – would never allow that. An open contest is a welcome sign of the party’s commitment to democracy and dissent. It also firmly demonstrates that Momentum is not to be a plaything of Loto. The organisation will surely start outflanking the leadership on policy, too.

Without Unite, Labour would still be run by its discredited old establishment and wilting like its European sister parties. Critically, both sides need to learn to have public differences without acrimony. Either candidate succeeding will mean a general secretary with strident leftwing politics who is committed to democratisation. And that’s surely the task ahead: the left often boasts that Labour is western Europe’s biggest political party, but most of the party’s membership is not actively engaged beyond paying their subs and voting in occasional elections. The party has appointed Dan Firth – a veteran of Citizens UK – to head up a community organising unit, an extremely overdue development.

Can the next general secretary build a democratic, pluralistic, transformative mass movement firmly rooted in local communities – even if that means the membership is sometimes on a collision course with the leadership? That’s the question, whoever triumphs in this contest.

• Owen Jones is a Guardian columnist