They must win the support of 5 percent of Labour’s constituency parties, or at least three of the party’s formal “affiliate” bodies including two trade unions.
This is not always a formality, as Emily Thornberry — now chair of the Commons’ foreign affairs committee — discovered when she failed to reach the local party threshold when running against Starmer in 2020.
Two of the four officials noted above suggested that Wes Streeting — despite being widely seen as a frontrunner — could struggle during this stage, as many unions and constituency parties regard his politics as not left-wing enough. A third official said this was inaccurate, given there has been an exodus of left-wing members from the party.
Step 5: Go to the party
Whoever pulls through the initial stages — probably three candidates at the most — will go through to a final vote of Labour’s members.
The result is difficult to predict, as many of the most left-wing members who joined under Jeremy Corbyn in the late 2010s have exited the party. In October’s deputy leadership vote, Bridget Phillipson — who was widely painted as being aligned to Starmer — lost by only a small margin.
The winner will be Labour leader, and therefore prime minister. If Keir Starmer wins, he will stay in No. 10, secure until the next time a critic is strong enough politically to come for him. If a rival wins, Starmer will visit King Charles III to tender his resignation as PM.
This story has been updated with additional details.