The gravitational pull of some political events is so big – the stakes so high or the subject matter so profound – that rules and alliances are broken or stretched out of shape. Though it may not seem as momentous as Brexit, the election of Jeremy Corbyn or Labour's 2015 defeat, Unite’s general secretary election is such an event for the internal politics of the Labour Party.

The result is a campaign that looks utterly peculiar. It began when Len McCluskey voluntarily resigned, exploiting a loophole in the union’s rules to allow him to stay on for a few additional years as general secretary. He was immediately supported by Momentum, which, without even consulting its members, urged them to join Unite. In the Momentum office there is seemingly a dial with only one setting: “join and vote for our guy”. His main challenger, Gerard Coyne, is likely to enjoy the support of Labour’s centrist factions in a way barely ever seen before in a union election.

Both sides of Labour’s divide recognise that the outcome of this election could determine the balance of power in the party. Under McCluskey, the union has aimed to make in-roads for anti-austerity and pro-union politics inside the parliamentary Labour party. In a poetic irony, it was the scandal around Unite’s attempts to win the selection battle in the Falkirk by-election in 2013 that prompted Ed Miliband to introduce One Member One Vote. It was this system of voting, of course, that allowed Corbyn to win.

Labour’s leftward shift, and Corbyn’s rise to the leadership, is overwhelmingly a product of the wider political weather, social movements, and the intellectual failings of the Labour right. But if it were to be attributable to any section of the labour movement’s machinery, it would be Unite. Karie Murphy, Unite’s candidate in Falkirk – who has been thoroughly cleared of any wrongdoing – now manages Jeremy Corbyn’s office.

Yet despite the pivotal role that McCluskey has played, his final election, the results of which will be announced in April, is potentially very tight. In 2010, the last election in which McCluskey faced a serious challenge from the right wing of the union, his two centrist opponents got a combined vote just 15,000 short of his winning tally. Add to that some of the votes which went to his other challenger at the time, plus a small backlash against Corbyn’s anti-Trident stance, and McCluskey could be in trouble.

That is a secondary consideration for Ian Allinson, a well-respected shop floor activist who led major strike action in Fujitsu. He is running on the perhaps dangerous assumption that there is no great risk of letting in the centrist Gerard Coyne by splitting the left vote. His candidacy also exposes the other great tension brought out by the Unite leadership election – that while Corbyn's supporters may be rooting for him, McCluskey’s politics are of a slightly different hue.

McCluskey is easily to the left of many union leaders – he has been proactive in building broad political movements like the anti-austerity protests of 2011, and has at times shown radicalism in Unite’s industrial strategy. But on the key thorny issues facing the Labour leadership, Corbyn supporters might find they have more in common with Allinson. Many Unite activists may share Allinson’s frustration with the union’s lack of mobilisation over the NHS, or its stance on immigration. When launching his campaign last year, McCluskey’s first policy was support for border controls in the wake of Brexit. Unite has played a role in influencing the PLP against free movement.

Unite’s support for Heathrow expansion and the Hinkley Point nuclear plant, as well as its triangulation on Trident, not only put awkward distance between its policies and Corbyn’s. They also point to an ideological division that in the past would have separated many Corbynites from McCluskey. On some level, Unite’s current leadership is committed to a "partnership" version of trade unionism, in which workers’ interests are defended by promoting the interests of industry, whatever the environmental cost or bigger picture.

Corbyn and his supporters know that they need trade unionists like McCluskey, and we are right to back his re-election with determination. It is inescapable, however, that the Unite general secretary election will shine a light on the gaps and disagreements that exist within the Labour left’s broad alliance. As the project rolls on, Corbyn will have to fight to maintain the breadth and unity of this coalition, but also for the radical policies that give life to it. The awkward question we must all answer is – on Trident, freedom of movement, and other key issues – what we do after winning yet another leadership election. On policies like these it is the labour movement’s grassroots, rather than union leaderships, that may offer the greatest hope.