Jon Lansman has electrified the race for the job of Labour’s general secretary by announcing that he will apply for the position. The role is elected by Labour’s 39-strong ruling national executive committee, making it an early test of where power lies within the Labour left.

What are his chances of getting it? He is thus far up against only one other candidate, Unite’s Jennie Formby, widely known to be the preferred candidate of the leader’s office. I’ve been speaking to various members of the NEC and other movers and shakers, and while his chances are not great, they are not non-existent either. But the more likely outcome is not that he will win, but that he could decisively tip the balance of the race against Formby.

The general secretary is elected by the NEC but the shortlisting process is done by the nine-strong NEC officers group, which is dominated by the major trade unions.

Allies of Formby pre-emptively briefed to the website Skwawbox that a Lansman candidacy carried two risks: the first of “splitting the left vote” and allowing a Corbynsceptic candidate to come through the middle, the second that if elected general secretary he would have to step down from the party’s NEC. Under the rules of Labour’s constitution, that results not in a by-election, but in the next best-placed candidate in the last election taking the post: so Eddie Izzard, a Corbynsceptic, would gain a seat on the NEC.

The second part is true although the left majority on the NEC is large enough that Izzard’s presence is not particularly important. But the first part is incorrect: the contest will be conducted by exhaustive ballot which means that it is impossible for a candidate to “split the ballot”. (Update: Skwawkbox say that their story was not briefed to them, and that the incorrect claim about “splitting the vote” emerged from them. I apologise.)

As I wrote earlier this week, anxiety in the trade union movement that the plum roles in the Labour party are all going to Unite officials is already high and Formby has left plenty feeling bruised thanks to her long career as Len McCluskey’s effective right hand woman. The important swing votes are the other trade union representatives on the NEC: the Transport Salaried Staffs’ Association, the Communication Workers Union, Unison, Usdaw and the GMB. It is likely that Usdaw will vote against Formby come what may, but the others are all up or grabs.

Privately, most agree with the Transport Salaried Staffs Association’s public statement that the next general secretary needs to be a woman and that it needs to be someone with impeccable leftwing credentials.

But “a woman” isn’t quite the same as “Jennie Formby”. If no other female candidate on the left of the party emerges, then you can expect that those members of the NEC will vote for Formby. But if another woman were to be found, then things would start to look different.

And that’s where Lansman’s decision to run is crucial. It was an astute move by Formby to announce her run so early as it prevented an alternate candidate arising from Unite and it locked in the endorsements of several members of the parliamentary Labour party. Lansman’s big achievement is in breaking the taboo against running for the post, something that may encourage others to follow suit. While people are rolling their eyes at Lansman’s call in his announcement for “more women” to stand for the post, it is very much in earnest. Opponents of Formby, who believe she will continue the old machine style politics, believe that the existence of a true-blue Corbynite opponent to Formby in the shape of Lansman will encourage other leftwingers to stand. (Others, who are less committed to it being a woman, are trying to encourage Paul Hilder, formerly of the Bernie Sanders campaign and who applied for the job last time, to put his hat in the ring as well.)

But Formby’s allies may have a crucial trump card: while the final vote is done by the whole of the national executive committee, the shortlisting is done by the NEC officers group, where Unite has three members of the eight-strong group. Often in Labour politics, the hand that controls the shortlist controls the world and it may be the same as far as the battle to replace Iain McNicol goes, too.