The powerful Unite union will call on Labour MPs to vote with the Tories to back the renewal of the Trident nuclear missile system next week.
The policy was passed by a large majority at Unite's conference in Brighton on Monday. Around 30 delegates spoke in a lengthy debate.
General secretary Len McCluskey put forward a statement from the union's executive committee. Like existing policy, it supports nuclear disarmament "in principle" but prioritises the defence of Trident jobs.
McCluskey welcomed Labour leader Jeremy Corbyn's call for diversification of Trident jobs and argued for pursuing this as a long term goal.
But he said, "In 2016, a vote not to renew those submarines is a vote to destroy those jobs. We can call for measures to protect them, but under this government that will not happen."
Several speakers called this a "fudge".
Vanessa O'Sullivan said, "A principle is not a principle unless you stick to it". She said the executive position "gives a free pass to right wing politicians".
Dave Kirk said it was a recipe for passivity. "It says we can't act on our opposition to nuclear weapons until we get a government that gives a 'cast iron guarantee' on jobs. But we won't get such a government unless we get fighting now."
Supporting the executive, many workers spoke of the devastating effect of job losses. Others said that Unite is "a union not a political party"—though McCluskey rightly insisted that political campaigning is important.
Steve O'Connell said the executive statement would damage Corbyn. He said, "It will be used by our enemies and Jeremy's enemies."
But McCluskey said, "This isn't about stabbing Jeremy in the back. Do you think for a moment that I would do that? Nothing could be further from the truth." He spoke of "long discussions" with Corbyn, who "understands our position".
McCluskey's opening speech, stressing that “unity is the most important thing”, hinted at the reason for the compromise.
Delegates from the Trident workforce made this overt, warning of potential mass defections to the GMB union. One said members "are asking for a clear signal they are welcome in this union". Another called anti-Trident motions "the biggest possible recruitment ad for GMB".
McCluskey echoed this, accusing GMB of "taking an opportunistic approach".
He said, "Shop stewards have had members asking, 'Is our union on our side? Because that GMB seems to be'. That's the only thing that matters to me - being on the side of our members."
Steve Sweeney pointed out that the 2014 conference's "brave decision" to oppose fracking hadn't stopped Unite organising fracking workers.
Many speakers argued against counterposing jobs and disarmament. Defence worker Brett Davis said, "Trident is not a job defender, it's a job destroyer".
He explained, "The defence sector is shrinking massively. Supporting Trident doesn't address that decline - it's only about managing decline. If we are serious about jobs we need to put ourselves in the vanguard of fighting for solutions for the 21st century.
"We need investment in productive sectors of the economy - and we can't do that while Trident is sucking up money and skills."
Chris Trestrail pointed to former defence workers who now make train carriages. He said, "If we don't make a case that our skills can be useful, we will just get attack and attack and attack."
Other delegates talked about renewable energy.
For the first time the Labour Party leadership supports such diversification. Brett said, "Opportunities like this don't come often. We need to seize the day and get off the fence."
Fighting job losses and weapons of mass destruction are more important than inter-union rivalries.
Reversing the decline in union membership would take bold, principled campaigning to win victories and inspire workers - not shoddy compromises to compete for existing members.