The Unite trade union, one of Labour's biggest backers, is set to call for Labour MPs to face mandatory reselection. This means MPs would face votes in their Constituency Labour Parties (CLPs) before being allowed to re-stand as Labour candidates for parliament.
Delegates voted overwhelmingly for the policy at the union's conference in Brighton yesterday, Thursday.
"These MPs have set themselves against our movement," argued Kevin Parslow proposing the motion. "We want to throw our members into the battle to make them accountable."
MPs consider it their automatic right to re-stand as parliamentary candidates. But Kevin said, "We put ourselves up for re-election as union reps - it's not unreasonable for MPs to have to do the same."
Seconding the motion, Mike Hedges, chair of Unite’s London and Eastern political committee, said, “Make no mistake, this will be a dirty fight, the establishment will stop at nothing”.
It followed speeches from shadow chancellor John McDonnell. On the conference floor he described how some MPs started organising against Corbyn's leadership from day one. He challenged them, "I'd rather be fighting Tories - wouldn't you?"
At a United Left (UL) fringe meeting he added, "All I can think is that the establishment is moving against us. And why? Because they can't stand a socialist leader who challenges the system."But in a question and answer session McDonnell discouraged deselections. "I think the majority of members now just want us to unite," he said.
The union's executive also had initially been set to oppose the measure. But amid anger at the intensifying attacks on Corbyn and those who voted for him they came out in its support.
At the UL meeting assistant general secretary Steve Turner said, "They are willing to sacrifice the 2020 election for the sake of their coup - if they get away with it. But we're not going to let them get away with it."
An earlier motion saw Unite back Corbyn's leadership. The debate showed the strength of support at the Labour grassroots.
Stewart Dack is vice chair of Reigate CLP. He said, "Our membership has tripled since Jeremy became leader. And how many people have resigned? One."
Former Labour member Darran Brown said he had left the party in disillusionment. "But having listened to Jeremy yesterday and John today I will be rejoining."
The vote at Unite conference follows the decision of CWU union branch in Bristol to stop supporting the city's anti-Corbyn Labour MPs. It could set further precedents.