A bitter battle over funding reforms could help keep David Cameron in Downing Street, Harriet Harman will warn trade union leaders tonight.
In a stark appeal for unity, Labour's deputy leader will insist that Ed Miliband is not seeking to weaken the historic ties between the party and the union movement.
The GMB has decided to reduce its funding of the party by over £1 million after Miliband said he wanted to make union members opt in rather than being automatically affiliated to Labour.
Other unions, including Labour's biggest affiliate Unite, have held back from following suit, offering breathing space to the Labour leader who makes a crucial speech to the TUC Congress on Tuesday.
Ms Harman will tell a dinner that she was "disappointed" by the GMB decision.
Accepting there were "some genuine concerns about the plans" - and acknowledging the party had wrongly ignored unions on issues such as council house building and agency workers - she will add: "There are difficult decisions that we have to work through ahead of our special conference next March.
"But how we go about it is important. We all need to think carefully about who gains from any falling out and any division. And we need to remember that fundamentally we're fighting for the same things - that we are on the same side.
"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. This is a dangerous moment.
"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.
"We must work together to shape and deliver that change."
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.Suggest a correction