Union leaders are being urged by Labour to unite behind Ed Miliband's plans to reform their historic links with the party or risk opening the door to a Tory general election victory.
Deputy leader Harriet Harman will issue the plea for unity as hostility to the proposed shake-up and the continued fallout from the Falkirk selection-rigging controversy dominate the annual TUC gathering.
"If we don't have unity, there are going to be winners and losers. The winners will be the Tories and the losers will be our constituents and your members," Ms Harman will tell them in Bournemouth.
A succession of union bosses lined up to attack Mr Miliband's proposals, which would see union members allowed to opt in rather than being automatically affiliated to Labour.
The GMB has decided to reduce its funds by over £1 million and Unison general secretary Dave Prentis revealed it would be making a £210,000 cut.
Other unions, including Labour's biggest affiliate, Unite, have held back from following suit, offering breathing space to Mr Miliband, who makes a crucial speech to Congress on Tuesday.
Mr Prentis said Labour would lose in 2015 "if it does not get its act together" - noting that internal "squabbles" were to blame for the heavy defeat of Australia's Labour Party at the weekend. He revealed that the number of members Unison affiliates to Labour, in a system similar to the one championed by Mr Miliband, is set to fall from half a million to 430,000, costing Labour £210,000.
Ms Harman will say she was "disappointed" by the GMB decision. She will recognise "some genuine concerns about the plans".
"But how we go about it is important. We all need to think carefully about who gains from any falling out and any division," she will say. "And we need to remember that fundamentally we're fighting for the same things - that we are on the same side. The change that Ed is proposing is not to weaken the relationship between Labour and the trade unions - it is to make it a reality."
She will tell them: "Your members and our constituents are feeling under pressure from all sides. The cruelty of the bedroom tax. The insecurity of zero-hours contracts. The worry that their wages stagnate as prices keep rising. Their need for us to be strong and united has never been greater. And they want just one thing from us - to kick this Government out. So whatever our differences - that is what we must do."