Parliamentarians versus party members? Leadership selection systems in the British Conservative and Labour parties

British parties have pioneered the use of 'one-member, one-vote' ballots to select their leaders. However, the elections of Jeremy Corbyn (Labour) and Liz Truss (Conservative) prompted calls to return leadership selection to parliamentarians. Critics claim that party members are non-centrist and liable to impose unsuitable leaders on MPs. This weakens the cohesion of parliamentary parties, undermining the functioning of Britain's majoritarian democracy. This article assesses the major parties' leader-selection systems. It goes beyond existing research by identifying and applying four evaluative criteria for selection institutions: legitimacy, parliamentary acceptability, leader-eviction and timeliness. It shows that most criticisms of one-member, one-vote are overstated because the latter is heavily mediated by ex-ante and/or ex-post parliamentary controls, for example, nomination thresholds and confidence votes. One-member, one-vote generally produces leaders acceptable to MPs; 'unsuitable' ones typically arise when the parliamentary controls fail. However, key institutional weaknesses are identified: legitimacy in the Conservatives' system and leader-eviction in Labour's.