Strong performance will be Shearer's only tool in riding out Cunliffe challenge.
With an address to the Labour Party conference that earned a rapturous response at the weekend, David Shearer's leadership should have been settled. Unfortunately he has gone a step further, seeking the endorsement of his caucus today.
He will probably get it, but if he imagines the vote will see off a challenge from David Cunliffe he is already disappointed. Mr Cunliffe declared yesterday he would support the leader in an immediate vote but reserved his position in February when party rules will force the issue again.
The prospect of a vote this week has served only to sharpen Mr Cunliffe's public comments. Mr Shearer now needs to respond to overt disloyalty, possibly by banishing his rival from the front bench, which would do the challenger no harm at all. An "exile" is always in a better position to criticise the leadership and become a focus of discontent.
A more experienced leader would have dismissed any suggestion he should try to "call out" a challenge with an early vote.
When a leader wins - as usually happens the first time - the question does not go away. It merely leaves the party divided and ensures the discontented faction will choose its moment to make another bid.
Challenges to an opposition leader are always corrosive. The only way to survive them is to perform as well as Mr Shearer is said to have done at the weekend and hope polls reflect the effort.
Today's caucus vote is also a response to the change made to Labour's constitution at the conference. Mr Shearer is asking for the endorsement of more than 60 per cent of Labour MPs, the test a leader will have to meet after every general election under the newly adopted rule that will take effect in February. Mr Shearer does not want the country spending a summer discussing whether he has enough votes.
His supporters at the conference believed the rule was motivated by discontent with his leadership but, if so, the rule could have been tougher. A 60 per cent caucus endorsement seems rather low to be a test of leadership of a major party. Neither main party has previously put a figure on the confidence level its leaders should be able to command, but close to 40 per cent dissent seems too much.
Mr Shearer should easily clear the 60 per cent threshold today, if only because the caucus will not be in a hurry to have its leader chosen under the rules adopted at the party conference. The next leader will be chosen by an electoral college in which party members and affiliated unions together could outvote MPs.
This could produce Labour leaders of quite a different complexion, with an effect on New Zealand politics that might not be to the party's advantage. It is odd that Labour has adopted this system so soon after it has seen a political party in the United States suffer for the influence of its extremes. The Republican Party lost not only the presidential election this month but also several senate contests because its primaries had alienated the party from too many voters.
Labour ran a public primary of sorts for its leadership last year. Mr Shearer and Mr Cunliffe toured the regions and made their pitch to members but in the end only the caucus had a vote. Mr Cunliffe might have made the stronger impression on the activists and policy enthusiasts in the party but the MPs preferred Mr Shearer. Having called for their endorsement today, he needs much more than 60 per cent to support him if Mr Cunliffe's challenge is to be not just beaten but buried.