LONDON — Jeremy Corbyn is finished as Labour leader — the fight for his legacy has just begun.

The unlikely poster child for the Left, whose expectation-defying performance in the U.K.'s last election in 2017 caught eyes across Europe and won fans in the United States, lost big Friday.

The disaster was apparent as soon as polls closed and broadcasters published an exit poll, which predicted a massive Conservative majority of 86 with Labour shedding dozens of seats including in its northern heartlands.

Labour's biggest problem had been clear to candidates who were given hell from voters on the doorstep throughout the campaign: Corbyn. But the party's high command argued otherwise.

“Brexit has dominated,” Shadow Chancellor John McDonnell told the BBC, minutes after the survey was announced. “We thought other issues could cut through and there would be a wider debate. From this evidence, there clearly wasn’t.”

When the election result became clear, one senior Labour activist summed up what would happen next: “Civil war.”

The Labour left was not scrabbling to save the blushes of Corbyn. Rather, it was seeking to frame the inevitable battle for the soul of the party in its own terms. For decades, the left longed to hold the levers of power in Labour, and it will not release its grip without a fight.

Blaming the historic drubbing on anything other than the left-wing leader and his policy platform was a no-brainer. The Labour left needed a scapegoat, and Brexit, an obvious factor in the defeat but far from the whole story, was it.

After the 2017 election, in which Corbyn deprived Theresa May of a majority, many critics of the leader in the centre and centre-right of the party sucked up their differences and put on a united front. Despite splits over Brexit deepening the rifts in the shadows, Corbynism became a vast umbrella under which various factions could sit, claiming to be represented by the squirming, triangulating positions the Labour leader settled on over different issues.

But as the Corbyn umbrella disappears, many in the party expect the structure beneath it to shatter into pieces.

“About 20 years-worth of political change has happened to the party within about three years,” one official said. “The membership has changed, the power structures have changed, the make-up of the parliamentary factions have changed. The only thing that currently holds everything together is Jeremy.”

“There is a whole kaleidoscope now with different people vying for influence,” the official added. Indeed, there are rival factions within the leadership office, within Momentum, the activist group that propelled Corbyn to power, and in the moderate wing of the party.

When the election result became clear, one senior Labour activist summed up what would happen next: “Civil war.”

3, 2, 1, FIGHT!

With the stakes high, the battle for the future of the party is not expected to be pretty.

“The next leadership election is going to decide if the party is gone for good or not,” said one Labour aide. “It's going to be the most unpleasant internal election anyone's ever seen.”

Indeed, as McDonnell and other Corbyn allies sought to put their spin on the defeat, the other side of the debate loaded its guns.

“They’ll try to make Jeremy Corbyn the fall guy, but every member of the shadow cabinet owns this defeat" — a Labour MP

“Deliberately misreading the exit poll from McDonnell,” said veteran MP and longstanding Corbyn critic Margaret Hodge on Twitter. “If this bears out, this is the utter failure of Corbyn & Corbynism. There is no other way of looking at it.”

Siobhain McDonagh, another Corbyn critic who was named as one of the MPs mulling whether to quit the party earlier in the year, said the defeat was “one man’s fault. His campaign, his manifesto, his leadership.”

Another Labour MP who has been critical of Corbyn, but spoke on condition of anonymity, said: “They’ll try to make Jeremy Corbyn the fall guy, but every member of the shadow cabinet owns this defeat. The problem isn’t just Corbyn, it’s Corbynism.”

One aide said: “The result needs to wake enough of the party up to the fact that if you take a hard left approach you get hard left results.”

As the result was declared in his own seat of Islington North, Corbyn made clear he knew his time was up.

“I want to ... make it clear that I will not lead the party in any future general election campaign,” Corbyn said. But he emphasized he would not quit immediately — a move that will spark fierce debate among the warring Labour factions.

“I will discuss with our party to ensure there is a process now of reflection on this result and on the policies that the party will take going forward,” Corbyn declared. “And I will lead the party during that period to ensure that discussion takes place and we move on into the future.”

The comment echoes calls by his close ally Len McCluskey, the general secretary of the Unite union, a major Labour donor. Critics argue the leadership wants time to position its preferred successor for the job.

Some had mulled whether McDonnell might take over as an interim leader as another way to keep control over the next steps in the succession process, but he ruled out doing so in the wake of the exit poll. The shadow chancellor told Sky News he would not serve “either as a temporary or a permanent” leader if Corbyn stands down.

Ruth Smeeth, who lost her Stoke-on-Trent North seat after battling Corbyn over anti-Semitism in the Labour Party for years, urged the leader to quit immediately. “There is absolutely no justification for why he’s still there,” she said.

Former Justice Secretary Charlie Faulkner told the BBC: “We’ve got to have a leadership election and have it as soon as we reasonably can.” Hodge quipped about Corbyn's call for a reflective period: "I’ve reflected. You failed. Please stand down."

The coming race

The starting gun on the leadership race might not officially be fired on Friday — but hopefuls will ramp up their efforts to position themselves for the top job. Some have already started.

Shadow Foreign Secretary Emily Thornberry is said by rival aides to have a leadership campaign at the ready.

“We may be hurting tonight but we are not beaten,” she said in an emotional speech at her election count in Islington South. “Our fight is not over. Our fight is just starting. And we will fight with the same spirit we showed. We will fight with the same courage my friend Jeremy showed.”

"Our members will be hurting over this defeat and will not want to repeat this disaster" — Labour MP Clive Efford

Shadow Brexit Secretary Keir Starmer, another senior figure tipped to throw his hat into the race, also made what sounded like a leadership pitch at his count in Holborn and St Pancras.

McDonnell protégé Rebecca Long-Bailey, Shadow Education Secretary Angela Rayner and Corbyn critic Jess Phillips are also frequently tipped as possible contenders.

Some who may have been in the running have already been knocked out of the contest. Laura Pidcock, a shadow business minister who was tipped as a possible successor on the left, lost her North West Durham seat to the Tories.

The difficulty for the contestants is that Labour is an unpredictable beast. The 500,000-strong membership, many of whom joined the party in 2015 specifically to elect Corbyn into the top job, is pro-EU, pro-left and pro-Corbyn.

It means hopefuls who want to win the top job might have have a difficult dance ahead: convincing the membership they are allies of the Labour boss and that they will keep the party on a similar track politically (see Thornberry mentioning her "friend Jeremy"); promising to fight the Tories on Brexit; while also proving they can appeal to the wider public.

As with the election of Corbyn, there is always room for surprise. “We are in an arena where if some candidate pitches it right and gets the mood of the party you could see quite an interesting result,” one official said.

Whoever pitches for the leadership, their main priority must be winning the next election — something many believe does not motivate the hard left enough.

"Labour’s problem is not the ideals and aspirations of the vast majority of our membership, it is those who hijacked their desires who don’t give priority to winning a majority in parliament," Labour MP Clive Efford said. "Our members will be hurting over this defeat and will not want to repeat this disaster."

One aide drove the point home by quoting a biography by Ben Pimlott of former Labour leader Harold Wilson.

“An opposition leader in the British system has many tasks: to keep the faith, inspire the troops, maintain unity, give a policy lead,” the book reads. “The most important is to win. Everything depends on the simple, clinical achievement of a House of Commons majority.”

Corbyn defied many of the rules in British politics. This isn't one of them.