How a party leader is actually chosen

Nobody in Britain votes for a prime minister. They vote for a local MP, and the leader of whichever party can command the Commons walks into Downing Street. That makes the internal rulebooks of the two big parties a piece of constitutional machinery in their own right, and both rulebooks read like scar tissue: every threshold, ballot stage and eligibility clause was written in reaction to some past disaster the party swore never to repeat.

The Conservative process is the more theatrical. A contest opens either when the leader resigns or when the 1922 Committee, the body of Tory backbenchers, receives letters demanding a confidence vote from 15% of the parliamentary party and the incumbent loses it. The 1922's executive then sets the rules fresh each time, including the nomination threshold, which is a live weapon: in October 2022 it was set at 100 MPs precisely to shrink the field to almost nothing, and Rishi Sunak became leader without a single member voting. When more candidates do clear the bar, Tory MPs hold successive exhaustive ballots, knocking out the last-placed candidate each round until two remain. Only then do the party's members, around 170,000 people, get a postal and online vote on the final pair.

That two-stage design dates from William Hague's 1998 reforms, and it was itself a correction. Until 1965 Conservative leaders simply "emerged" from soundings among grandees; then MPs alone elected them, which produced Thatcher and later the coup against her. Handing the last word to members was meant to democratise the party. Instead it produced Iain Duncan Smith in 2001 and Liz Truss in 2022, both preferred by members over the candidate MPs favoured, and both gone without fighting a general election. The current thresholds exist because the parliamentary party keeps trying to claw back the gatekeeping it gave away.

Labour's fossil record is older and deeper. Until 1981 its MPs chose the leader alone. The Bennite insurgency forced through an electoral college splitting the vote between unions, MPs and constituency members, a machine so baroque that Ed Miliband beat his brother David in 2010 despite losing among both MPs and members, on the strength of union-section votes. The Collins Review scrapped the college in 2014 for one member, one vote, with a £3 "registered supporter" category attached. That door swung open exactly once: Jeremy Corbyn, lent MP nominations by colleagues who wanted a token left candidate, won among the enlarged selectorate in 2015 and held the party for four years.

The thresholds are the story

Every rule change since has been about that door. Corbyn's allies cut the MP nomination requirement to 10% in 2018; Keir Starmer's leadership pushed it back up to 20% in 2021, and added a requirement that candidates also gather nominations from 5% of constituency parties or from affiliates including at least two unions. On current parliamentary numbers, 20% means a candidate needs roughly 80 MPs before members see a ballot paper. The membership still formally decides, but the parliamentary party now controls who can be decided upon, which is the college restored in all but name.

The smaller parties show the same pattern at lower stakes. The Liberal Democrats have used a straight member ballot since their foundation, with candidates needing the backing of 10% of the parliamentary party and 200 members across 20 local parties. The SNP ran its first genuinely contested membership ballot in decades in 2023, and the closeness of Humza Yousaf's win over Kate Forbes exposed how little the party machine knew about its own selectorate.

Why the mechanics decide the outcome

The practical effect of all this is that British leaders are chosen by two selectorates in sequence, and the order matters. MPs optimise for someone they can work with and the country might elect; members optimise for conviction. Where MPs filter first, as in the Conservative system, the members' choice is confined to candidates the parliamentary party can live with, at least in theory. Where the filter is a raw numeric threshold, as in Labour's, the fight happens before the contest even starts, in the arithmetic of nomination-gathering. A candidate who cannot reach 80 colleagues, or survive successive MP ballots, never reaches the country at all. The rulebook, not the hustings, is where most leadership contests are actually won.