Rift fuelled by anger at Labour leader's refusal to back public pension strikes and the sense he is abandoning those who elevated him
Brewing discontent with Ed Miliband among trade union leaders was evident last year at their annual gathering. And it manifested itself in disrespect.
As Miliband entered the main hall at the Trades Union Congress, one senior union backer made a dismissive comment, switching to a friendly greeting as the Labour leader approached.
More bravado than outright rebellion, those little displays of irritation have become a full-blown attack now. If Miliband dislikes being mocked behind his back, at least he has the consolation that the salvos are coming from the front now.
The TUC gathering in September was a galvanising moment for the union movement, where organisations representing 2.6 million public sector workers affirmed their determination to hold a joint day of action over pensions reform.
The atmosphere was necessarily hostile to the government, because industrial action always takes place in an attritional environment. As a result the emotional stakes are higher, also due in part to the political risk: unions were defining their movement in the eyes of the public with these strikes. It was their big moment in the public eye. At this point the choice facing Miliband was not one nuance or hedged turn of phrase over another; it was Manichean: in or out.
And Miliband, his eyes on a floating vote that now appears to have gone from peripheral to central vision, chose to be out. Describing strikes over pension reforms as a "mistake", he was heckled and jeered, confirming a rift with unions that now threatens to become outright divorce.
There is a sense that Miliband is abandoning, or being shunted off, the political stance on which he campaigned for the Labour leadership. If the TUC "mistake" moment was a piece of awkward positioning, then the Ed Balls speech at the weekend was viewed as something of much greater magnitude: an about-turn, evidence of a Blairite resurgence.
Len McCluskey's hostile letter is the most aggressive attempt yet to keep Miliband in the union camp. After all, unions need Labour's political power as much as the party needs their financial support. Despite the GMB's strong words on Monday, Unite is not actively considering disaffiliation yet.
But there is clearly a view emerging among the labour leadership that the public is declining to join, at least in emotional terms, a bedrock of political support that contains 2.6 million public sector workers. If the Conservatives and the Liberal Democrats have the pro-austerity vote sewn up, say the likes of Unite and the GMB, then Miliband is in danger of casting his party and the union movement into the political wilderness.