One of Labour's 2014 election candidates has written a stinging open letter to David Cunliffe, saying he would leave the party if Mr Cunliffe won the leadership contest and challenging Mr Cunliffe to promise to step out of politics if he lost.

James Dann was Labour's candidate in Ilam and wrote a guest post on the Public Address website, saying that during the election campaign a number of people he door knocked had told him they would not vote Labour while Mr Cunliffe was its leader.

Others had refused to put up hoardings with Mr Cunliffe's face on them.

Mr Dann said in the letter that he had supported Mr Robertson in the last leadership contest but after Mr Cunliffe won he had swung in behind, including organising Labour's annual conference soon after, driving Mr Cunliffe around during the Christchurch East byelection, and giving him credit for a strong personal campaign.

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However, he said he would not be part of the party if Mr Cunliffe was leader.

"If you win, I'll step aside from the party, to let you and your supporters mould it into the party you want. But in return I ask this: if you lose this primary, you resign from parliament."

He said Mr Cunliffe had destabilised leaders before him in order to get the position. "Then when you got your chance as leader, you led Labour a party that was polling in the mid-30's to one that sits firmly in the mid-20's. There is no place for you in this party anymore."

Mr Dann said Mr Cunliffe had had his shot.

"The Labour Party isn't a vehicle for you to indulge your fantasy of being Prime Minister. While you might think that it's your destiny to be the visionary leader of this country, the country has a very different vision -- and it doesn't involve you."

The open letter is an indication of how polarising Mr Cunliffe's decision to contest the leadership has been and how much more fractious this leadership contest is likely to be compared to last year's.