A little-noted section of the GMB's motion to investigate Progress was not about the New Labour pressure group at all.
It was about one of its donors.
The section read: "Congress notes that the principal Progress patron is Lord David Sainsbury who has been a donor since April 2004 and continues to fund Progress at the rate of £260,000 a year. His donations of over £8.4million to the Labour Party stopped when Ed Miliband became leader."
Now, Lord Sainsbury was one of Tony Blair's biggest supporters. He also offered help to David Miliband during the leadership elections.
The GMB are right to note that he has not donated to the party since Ed became leader.
But he still donates to Progress (and also to David Miliband's baby Movement for Change).
And as one Labour MP noted: "It's strange when the fact that someone is a New Labour donor and supporter becomes an attack."
They added that Labour-affiliated trade unions similarly express a preference for certain Labour Party candidates and MPs.
"You are allowed to be a Labour Party supporter and still express preferences for certain wings of the party."
Progress responded to the GMB's motion:
"There is no relationship between the funding we receive from Lord Sainsbury and the funding that he gave to the Labour party. Our annual income from him was £250,000 until May 2010, when it was raised to £260,000. This was before he ceased giving money to the Labour Party and our income from Lord Sainsbury has not been increased (as the resolution implies) as a result of that decision."
Yesterday, Lord Mandelson waded in on the dispute.
He told the Andrew Marr Show: "Trade unions need to rethink and remake themselves for a new century.
"If they become, in a sense, more representative of their membership as a whole, they would not be leading either themselves or the Labour Party down what I regard as a pretty blind alley."
This is unlikely to be the last of it. While Ed Miliband has tried to calm the storm, divisions from Labour's time in government go deep.
If they can't unite behind Ed - who is pushed as a 'consensus' figure in the party - can the tensions ever be healed?