"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.