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 of either five per cent of Constituency Labour Parties – 32 CLPs – OR five per cent of affiliates – including two trade 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 trade 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 trade 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 opt to endorse her 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 shadow Brexit secretary Keir Starmer and shadow foreign secretary 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 backbenchers Jess Phillips and Lisa Nandy, who are pondering longshot bids, and both of whom will need the support of CLPs to reach ballot. There is little prospect that either candidate will be able to make the ballot paper via the union route – in the case of Nandy, because she lacks institutional support, and in the case of Phillips, 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.