How important are the trade unions in the Labour leadership contest?

The race to succeed Jeremy Corbyn as Labour leader is already well under way, despite the contest not officially beginning until early next year.

Behind the scenes, potential candidates are holding hushed conversations with Labour MPs, campaign teams are being assembled, funding secured, and alliances struck.

One of the key challenges facing the prospective candidates is winning the support of the 12 trade unions affiliated to Labour and, more importantly, that of their members.

Unions play a key role in Labour leadership elections, largely because, in addition to party members, members of affiliated trade unions are also entitled to vote, providing they opt in.

And while unions cannot vote on behalf of their members, formal union backing is seen as crucial to giving a candidate legitimacy.

It has long been the case that candidates were not seen as entirely credible unless they could secure the support of at least some of the unions that are a key part of the labour movement.

But recent changes to party rules mean the unions are now even more important.

Under reforms made last year, candidates hoping to stand for the party leadership must now win the support of either 5 per cent of local Labour branches or three affiliated organisations, of which at least two must be trade unions. It means that for candidates unsure of securing nominations from constituency Labour Party (CLP) branches, union support may be their only hope. Candidates must also be backed by 10 per cent of Labour MPs and MEPs combined.

As well as endorsements, unions also play a key role in funding candidates’ campaigns. They often provide office space and other resources to favoured candidates, as well as cash donations and loans.

For all these reasons, the array of Labour leadership hopefuls will be cosying up to union general secretaries and political officers over the coming weeks. But who is most likely to succeed?

In total, 12 trade unions are affiliated to Labour. Of these, the three biggest and most influential are Unite, Unison and the GMB.

Unite is traditionally the most left wing. Its outspoken general secretary, Len McCluskey, is a staunch ally of Corbyn and has been a long-standing member of his inner circle. The union is expected to endorse Rebecca Long Bailey, the shadow business secretary, given she is seen as the candidate most likely to continue the Corbyn project. Ms Long Bailey would also be expected to win the backing of other left-wing unions such as the RMT and TSSA, which both represent transport workers.

The second biggest union, Unison, is less radical and typically backs more moderate candidates, although it supported Corbyn in 2015 and 2016. Both Keir Starmer, the Brexit secretary, and Emily Thornberry, the shadow foreign secretary, will be hoping to secure its support, although it is possible that it too will back Long Bailey. Lisa Nandy, the Wigan MP, could also be in with an outside chance if she performs well early in the campaign.

The third biggest union is GMB. It is unlikely to back Thornberry but could opt for either Starmer or Long Bailey. GMB worked closely with Starmer in the campaign to convince the Labour leadership to support a second referendum.

A key factor in determining who unions support is the timetable for the contest. A sense of momentum is crucial in leadership elections and if, as expected, MPs nominate candidates before unions and other affiliated organisations do, that is likely to influence who the unions decide to back. They will want to choose someone who is at least in with a chance of winning, and candidates who perform poorly among their parliamentary colleagues risk missing out on key union endorsements.

In this sense, MPs are even more influential during the early stages of the leadership contest than the union bosses. Ultimately, though, the contest is all about members of the party and the unions. It is they who form the electorate, and they who the candidates will put most effort into convincing.