The rise of Jeremy Corbyn has, it goes without saying, completely shifted the terms and nature of the Labour leadership debate. The panic in much of the Labour Party elite is palpable – whether it is Tony Blair calling for Corbyn supporters to “get a heart transplant”, or Margaret Beckett calling herself a “moron” for nominating Corbyn, the party’s upper echelons now seem to be growing increasingly concerned.

And with the first YouGov poll showing Corbyn ahead in the contest, some have given into the temptation to call for a more united anti-Corbyn front. With Liz Kendall in fourth, the logic runs, she should quit the race. Her supporters, on the right of Labour, would move to the more moderate Andy Burnham and Yvette Cooper, giving them an edge against the further left Corbyn, in essence creating a ‘Stop Corbyn’ element of the race.

Yet, in reality, this is a thought process for a First Past the Post (FPTP) system used in UK general elections, whereas the Labour leader is elected under the Alternative Vote system.

Under FPTP, the candidate with the most votes wins, but this can result in candidates acting as a ‘spoiler’. In this example, the theory would run that Kendall would be ‘taking’ votes from her more moderate competitors. But in the preferential AV system, the winner is only deemed to have ‘won’ if they secure more than 50% of the vote.

Voters rank candidates in order of preference, and if no one wins a majority of first preference votes (defined by a ‘1’ on the ballot paper) then the worst performing candidate is eliminated and votes for that candidate are given to the second preference of their supporters (a ‘2’ on the ballot). If no one has 50% of the votes again, the worst performing candidate is eliminated again and this process carries on until someone reaches 50% support.

Because of this, Kendall’s supporters will already move to other more centrist candidates (overwhelmingly Cooper, followed by Burnham), in the event, as the YouGov poll suggests, that she comes fourth and is thus eliminated first. 55% of Liz Kendall’s supporters tell YouGov that they will give Yvette Cooper their second preference, 22% Andy Burnham, and 6% Jeremy Corbyn (the small number remaining either haven’t decided or haven’t expressed a choice). If she were to withdraw, we can also expect that this is where her supporters’ votes would go.

Ultimately, Corbyn’s triumph in the YouGov poll is because he starts out with a commanding lead in first preferences (43% vs. 26% for Burnham) and then wins enough second preferences from Yvette Cooper (22%) to win a majority of votes in the final round, by a 53% to 47% margin. Hence, he is, if this poll is to be believed, preferred by an absolute majority of those voting in the contest. Candidates withdrawing will do nothing to change this fact as long as the current views of their supporters remain the same.

First Past the Post-thinking misunderstands the nature of the rules of the game. To beat Corbyn, the three other candidates need to persuade those voters giving him their first and second preferences to give them to one of Burnham/Kendall/Cooper instead. Telling candidates to drop out misses the point entirely – one of the benefits of AV, though it’s not perfect, is that you don’t have to.

Most parties are wise enough to have realised long ago that ‘splitting the vote’ is a dangerous game for internal party elections. As we’ve long pointed out, it’s a dilemma mostly confined to council and Westminster elections. Perhaps it’s time for the candidates to talk about changing that…

Chris Terry is a Research Officer at Electoral Reform Society