LIVERPOOL, England—Jeremy Corbyn’s election as leader launched a left-wing revolution inside the Labour Party. Centrist Labour politicians have utterly failed to slow the march toward socialism, but this week the trade union movement stepped in to say: Let’s not get carried away.

Efforts to radically change Labour Party rules to encourage more left-wing politicians and a bid to steamroll through the left’s Brexit position without debate were both halted by the major unions.

One of the union bosses responsible told The Daily Beast that the needs of millions of workers trumped the desires of a smaller ideological insurgency.

The response on the floor of Labour’s annual conference was explosive, with exasperated activists furious that the unions had kiboshed plans for “open selections” for Labour parliamentary candidates. Boos rang out and there were shouts of “Shame on the unions!”

Those plans had been devised by the radical grassroots movement Momentum, which backs Corbyn and is determined to push Labour ever further to the left.

The unexpected outbursts forced Labour officials to plead with delegates not to boo and disrupt what is seen as the party’s biggest annual showcase. Outside the conference hall, debates over the direction of the party raged into the night in the bars and packed fringe events hosted by rival factions.

Momentum, which grew out of the campaign to elect Corbyn in 2015, now has around 40,000 members, which makes it a hugely powerful force inside Labour. By way of context, the governing Conservative Party is thought to have around 70,000 members in total.

Its founder, Jon Lansman, a veteran left-wing campaigner with bright white hair, was elected to Labour’s ruling body, the NEC, this year. His bid to become Labour general secretary faltered when a trade unionist was anointed by Corbyn—a first indication of the split between Momentum and the unions—but Lansman remains one of the most influential left-wing figures in Britain.

In the lobby of the conference hotel, he was frustrated. Lansman denied that Momentum was “at war” with the unions, but admitted he had been disappointed that the major unions had agreed to a deal to block Momentum’s plans among themselves before a big NEC meeting ahead of the conference.

“People have different views,” he told The Daily Beast. “It’s understandable that unions wanted to be together, but it would have been better to make the decision at the NEC. But we understand their desire to move together.”

In the end, Lansman had urged Momentum members to back the compromise deal proposed by the unions. “If we’d waited, we might have got nothing,” he explained, although he emphasized that this did not quell the anger of his supporters on the floor. “We saw the strength of the feeling from the grassroots.”

Indeed Lansman was heckled himself at one evening event for accepting the compromise, which raises the possibility that even Momentum’s founder may not be able to control the insurgency he has unleashed.

It wasn’t just the grassroots members who were upset by the unions’ move—Chris Williamson, arguably Labour’s most left-wing MP and an ally of Corbyn, was spotted in a heated argument with union boss Len McClusky after Williamson had publicly attacked him for changing his mind over “open selections.”

Lindsey German is another friend of Corbyn; they set up the Stop the War campaign together to campaign against President George W. Bush’s “War on Terror.” The former Socialist Workers Party activist is an influential voice who shares Corbyn’s controversial world view. She told The Daily Beast the unions had taken fright at the speed of the revolution. “I suspect most of them would like to see a change in some of the MPs, but I think they’re worried it would rock the boat too much,” she said.

Steve Howell, a former Corbyn adviser whose book Game Changer gave the inside story of last year’s Labour’s election campaign, said the skirmish between Momentum and the unions was the only blemish on a fine conference. “It’s unfortunate that the dividing line appeared to be individual members versus trade-union members. I do think that it’s important that both sides of the party communicate better with each other.”

Away from the beer-soaked evening bashes, the head of the GMB general workers union explained why he and colleagues at the other major unions had decided to halt “open selections.” The idea, devised by Momentum and pushed at conference by sympathetic constituency labour parties (CLPs), was to introduce a U.S. primary-style system so that incumbent MPs could be more easily replaced. The left feels it is necessary because so many moderate Labour politicians have criticized or even tried to oust Corbyn as party leader.

“The CLPs represent 40,000 people, but the trade unions represent millions of people. People need to just recognise that,” said Tim Roache, general secretary of the GMB, speaking to The Daily Beast at an event on socialism for the 21 st century.

“Open selections all the time would create the sort of MPs that we wouldn’t want. MPs would spend all their time in the constituency looking after the people that could trigger them and not do the stuff in the wider community and in the parliament that they would need to do.”

Roache’s union officials were also instrumental in defeating the Labour leadership and the left of the party who wanted to avoid a debate with the vast majority of party delegates who are stridently anti-Brexit. The left, by contrast, views the EU as a club dominated by financiers with an built-in hostility to socialism.

The GMB union’s Brexit position is nuanced and Roache is aware that a large percentage of his members feel it should not be overturned; but ultimately, he sided with party moderates and helped change Labour Party policy to force Corbyn to fight harder against Theresa May’s position.

“My job is to ensure that the world is a better place than when I inherited it and better place for my kids. And I just think we’re heading over the cliff and I’m trying to do something about it,” he said.

For a generation, the unions have been a force pulling Labour to the left of the political spectrum—the fact that they have been a voice of moderation this week in Liverpool shows how far the party’s center of gravity has shifted since Corbyn took control.

This ideological ebb and flow has been a regular theme of the history of the Labour Party, which was founded from of the trade union movement more than a century ago. The unions have stepped in numerous times when radical left-wing factions threatened Labour leaders such as Clement Attlee in the 1930s and ’40s and Hugh Gaitskell and Harold Wilson in the 1960s.

This time is different; the left is not just a growing influence on the party, some of its most radical proponents are at the helm. Corbyn’s leadership office is staffed by anti-war campaigners, activists, and former communists, while his old comrade John McDonnell is setting the party’s economic policy.

Not all of Britain’s trade unions share the desire to moderate the instincts of Momentum and the leadership. Unite, the largest union affiliated to Labour, is a staunch supporter and many of the smaller, independent unions share their views.

Some of the radical unions gathered in a tiny room just outside the conference secure zone on the banks of the River Mersey this week for an event on how Labour can best work with “the fighting trade unions.”

Ian Hodson, the charismatic president of the Bakers, Food and Allied Workers’ Union, opened proceedings with two bottles of beer placed on the table in front of him. By the time he had drained the second, McDonnell had arrived.

Corbyn’s righthand-man, a slight and softly spoken politician, is destined to become the Chancellor of the Exchequer if Labour wins the next election. He’s been one of the biggest draws of the conference, laying out the party’s economic program to packed halls.

He’s also a hardened left-winger burnished by decades of fighting alongside socialists, radical trade unions, and Irish Republicans.

Hodson nodded his appreciation as McDonnell entered the room of fewer than 100 people, but unlike at other events the busiest man at conference wasn’t beckoned straight up on stage. Here he was among real, close friends and he nodded and chuckled along to another union speaker before he stood up and told the trade unionists in the room that when Labour won the election they would all go into government as well.

He asked for help in crafting the program of reforms that would be introduced in Labour’s first term, although he explained that not every desire would be possible in the first raft of changes.

“What I’ve always found with ordinary working-class people: We understand the language of priorities, we’re not asking for the earth. Every one of us is realistic about what can be done and how it can be done. From a Trotskyist position, I’m not expecting a whole series of transitional demands.”

Everyone laughed at the Marxist theory gag, which referred to demands made by the workers that could never be delivered without fundamentally damaging the capitalist system.

“There’s a headline for tomorrow,” bellowed Hodson. “'McDonnell quotes Trotsky. What a Trot!’”

Sitting amid this comradely atmosphere, it was obvious that the battle for the soul of Labour was far from over. The moderate unions may have won this week but their victory could be short-lived.