The final third comes from affiliated unions and an array of socialist societies. In an ideal world, of course, the leader has the confidence of these three distinct parts.
But the potential flaws of this approach were highlighted by its most recent use in 2010, when Ed Miliband was elected leader. Not only did he win by a narrow margin over his brother David, itself a recipe for lingering discord, but his victory was due in large part to a strong vote from the unions.
David Miliband was the favoured candidate with MPs, the men and women with whom the leader must work most closely. Worse, Ed Miliband's reliance on trade union support left him wide open to attacks from the Conservatives and other political opponents. Subsequent events have done little to alter the view that the leadership voting system led to the wrong man being elected.
Events in the United States are also illustrative. There, primary campaigns often throw up extreme candidates who garner a surprising degree of party members' support, only to fall away when serious decisions must be made.
All this reinforces the strength of the Labour Party's present procedure, as does the fact that MPs have a vested interest in selecting the right leader. If they pick the wrong person, they may well undermine their parliamentary tenure. They have also observed the candidates at the closest quarter. They know who they can work with and who is unpopular within the caucus, the precursor of an unworkable situation. Notably, the Labour caucus selected David Shearer, rather than David Cunliffe, who appeared to resonate more strongly with the party membership during the leadership contest.
At Labour's caucus retreat this week, the weaknesses of the British approach seem to have been acknowledged. It will not be adopted. But other means of creating the three-way split have their own difficulties.
The least problematic solution might be for MPs to retain somewhere between 45 and 50 per cent of the vote, with members having 40 per cent. Affiliates such as unions and the party's Maori and youth wings would hold the balance, except for 3 per cent that would be set aside for registered supporters who are not members. The latter, a further attempt to open the party to a wider audience, was adopted by the British Labour Party last year.
In such an arrangement, it would be important that the caucus portion of the vote was not a bloc. If this were so, the votes of party members and others would cease to have much meaning. The members' input would become token. In essence, the situation would be little different to that of list MPs, a subject of much popular discontent.
Labour's intention is correct.
But it will have to tread carefully in making the membership vote a meaningful part of the equation while also ensuring that a leader who is unpopular in the caucus is not foisted upon MPs. The potential rewards of its proposal are great but so, too, is the possible downside.
