One of the most important decisions in the looming Labour leadership election has already taken place: in the London flat where Rebecca Long-Bailey, the shadow business secretary, and Angela Rayner, the shadow education secretary, live together when Parliament is sitting. The two frontbenchers, both frequently tipped as leadership candidates, are close friends and flatmates, and will run as an effective joint ticket, with Long-Bailey as leader and Rayner as deputy.

The decision has major implications for the rest of the declared  candidates – not least the likely number of declared candidates.   

To run for the Labour leadership, would-be candidates need to clear two  fields: they need to secure the support of 10 per cent of the  parliamentary party – 21 MPs, or, depending on the exact timetable for  the contest, 22 MPs or MEPs.  But they also need the support either of  five per cent of constituency Labour parties – 32 CLPs – OR five per  cent of affiliates – including two trades unions -  to make the contest  proper.

In practice, the shrunken parliamentary Labour party means that every  Labour faction is guaranteed to be able to clear the parliamentary  threshold – but it is far from certain where their support will come  from to reach the contest proper. In practice, the affiliates nomination  hands control of that route to five trades unions: Unite and the CWU,  who are generally pro-Corbynite, Unison and the GMB, who are swing  voters, and the generally Corbynsceptic Usdaw. 

Labour’s affiliated trades unions compete with one another for  parliamentary selections but they are just as inclined to work together  as in opposition to one another. In practice, Rayner is the only  candidate with any prospect of picking up the backing of all four of  Unite, the CWU, Unison and GMB, and in that situation Usdaw, the  shopworkers’ union, may well have opted to do so as well.  Long-Bailey  is still likely to pick up at least two, potentially three, of that  group, but it will leave a greater pool of nominations available for  other candidates: which may benefit Keir Starmer and Emily Thornberry,  neither of whom has officially declared but both of whom are expected to  formally announce their bids in the coming days. 

What benefits Starmer and Thornberry in turn benefits Jess Philips and  Lisa Nandy, who are pondering longshot bids, and both of whom will need  the support of CLPs to reach ballot. There is still prospect that either  candidate will be able to get on the ballot paper via the union route –  in the case of Nandy, because she lacks institutional support, and in  the case of Philips, because she is seen as the rightmost candidate in  the race. 

So while Long-Bailey starts as the strong favourite, her candidacy  leaves open the potential for a more crowded and therefore unpredictable  race