When an opposition party chooses a new leader in the wake of defeat, the event has the potential to be a moment of rebirth: sorrows can be put aside, a line drawn under past failures, the party may dare to dream again.

It was like that for some in the crowd at the Queen Elizabeth II conference centre when it was declared that Jeremy Corbyn had become Labour’s crimson king by conqueroring the party with a haul of votes that obliterated his rivals. Cue much celebration by the forces that have – against all early expectations, including his own – transformed the career rebel into the leader of Her Majesty’s Loyal Opposition. A giddy red letter day for the gnarled old comrades who have waited for generations for one of their own to occupy Labour’s high seat. An exhilirating rush for the fresher recruits who were drawn to his banner.

Whatever your politics, it is a stunning achievement. Whatever happens next, to capture the leadership of the Labour party, and to do so at the age of 66 after more than three decades on the backbench margins, that is an extraordinary campaign triumph. His victory may in part be owed to the many mistakes made by Labour’s moderates, including his competitors for the crown.

Those who preached so often to their party about the necessity of winning general elections proved to be useless at winning a Labour one. It is as much testimony to the way he and his team organised their assault on the commanding heights of Labour. It was cleverly done: the candidate with the best machine successfully being sold as the antithesis of and antidote to machine politics.

The ecstasy of the Corbynistas was agony for one significant body of party opinion: Labour MPs. One of them, not a man usually given to hyperbole, grieved: “My party has just hurled itself off a cliff.” He clearly speaks for more than himself. No more than 20 Labour MPs – less than 10% of the parliamentary party – wanted this result. This takes us into terra incognita. For once, the overused words “seismic” and “unprecedented” are fully justified. Never before has a leader commanded so many votes from the selectorate and such scant support among his parliamentary colleagues.

On the day and on both sides of this schism, most people were calculating that it was in their interests to try to sound magnanimous.

The new leader peppered his victory speech with calls for unity. Even his bitterest opponents among Labour MPs are stressing that they accept the result. Earlier foolish talk about a rapid attempt to unseat him has evaporated. The thumping scale of his victory means that his opponents within will have to tread very carefully for the moment.

Yet the truth can’t be concealed. This leaves Labour MPs more divided than I have ever known them. Divided between those who are convinced that the Corbyn leadership will be an instant disaster and those who reckon it will be more of a slow-burning catastrophe for their party. Then there are those who candidly confess that they have no idea where their party is now going. “I think I know how this will end,” says one. “But I can’t say when.”

The struggle of wills may in part take the form of a competition of mandates. Mr Corbyn’s record-beating history of rebellion against previous Labour leaders doesn’t give him the moral authority to simply demand the loyalty of MPs as of right. What he can do is exploit the size of his victory to intimidate parliamentary colleagues and seek to fortify it by converting his many £3 supporters into full members. Some MPs will retort that they have another mandate, the one that they received from their constituents in May. In the words of one of their number: “I was not elected on a manifesto to withdraw from Nato.”

The immediate question – one that matters not just to Labour, but to the country – is whether the party can be viable as an opposition when there is such a vast chasm between its leader and its MPs. Some of Labour’s larger names pre-declared that they will not join his frontbench team, among them Yvette Cooper and Liz Kendall, two of his defeated competitors. Given what they have said about his fitness to be leader and his ideological positions, they could not really have done otherwise than decline to serve in his shadow cabinet. The refuseniks are unwilling to put themselves through the torture of trying to pretend that they think he is a credible candidate for prime minister.

“Anyone who sups with him will be permanently tainted. Their credibility will be destroyed,” predicts one former Labour cabinet minister.

It is indeed rather hard to see how any Labour politician with an aspiration to lead the party back towards the centre at some time in the future can do so if they sign up to the Corbyn prospectus. There is also the calculation among some Labour moderates that he ought not to be impeded so that when the project collapses, as they assume it inevitably must when it collides with electoral reality, it will be clear where the blame should be located.

“When he fails, it is essential that he and his fellow travellers own that failure,” says one member of the shadow cabinet who is going into internal exile on the backbenches. There are other Labour MPs who think they have to try to work with him for careerist calculations or because it is expected of them or to try to maintain some influence over the party’s direction. Tom Watson, the new deputy leader, loathes being labelled a fixer, but many are looking to him to do the fixing. He has rapidly cast himself as the bridge-builder and has been working hard at it.

I expect Mr Corbyn will get the co-operation of sufficient Labour MPs to populate his frontbench. Even so, this is going to be – I put it lightly – a very strained marriage. That is exemplified by the hideously compromised position of Andy Burnham. He has indicated a willingness to serve in the shadow cabinet, but has since been caught saying privately that this is “a disaster for the Labour party... the public will think Labour has given up on ever being a government again”.

The Tories are already planning to stage parliamentary votes designed to illustrate and amplify the differences between the Labour leader and the party’s MPs.

There have been some indications that he will try to make it easier for parliamentary colleagues to work with him by kicking some of the positions that fill them with most horror, such as withdrawal from Nato, into the long weeds of a drawn-out policy review.

Some of his friends say that they understand a “big tent” approach has to be adopted. But finding a modus vivendi will require a capacity for compromise that has not been the notable feature of a political career lived in a leftwing bubble. And he can’t make too many concessions to make his leadership more palatable to parliamentary colleagues without risking the alienation of the people who have just made him leader on the basis that he keeps his principles unpolluted by pragmatism.

No one can sensibly forecast exactly what sort of chief of the Labour tribe he will turn out to be because Labour has never had a leader anything like him and he has spent his entire career avoiding being anywhere near a Labour leader.

It was a great advantage during the contest never to have been sullied with any responsibility. Now he will have to make a dozen difficult decisions before breakfast. He will face a level of pressure that his friends acknowledge will be many, many multiples more intense than anything he has known.

There were moments during the campaign that revealed that he can be thin-skinned. He devoted an angry chunk of his acceptance speech to complaining about journalists. Showing that you can be wounded by the Tory press will only encourage them. There will be a massive onslaught from the Conservatives and their media allies. The widespread assumption in the Westminster village that he will be a calamity might, at least in the short term, work to his advantage if he can surprise expectations on the upside in his early weeks in the role.

“We shouldn’t behave as if all our Christmases have come at once,” says one Tory strategist, even if the truth is that the Conservatives absolutely do believe that they have been gifted an opponent beyond their wildest fantasies.

What greatly worries some Labour MPs concerned about the long-term fate of their party is that the Conservatives won’t confine themselves to target ting Mr Corbyn. The Tories’ strategic intent is to try to inflict reputational damage on Labour that will endure whatever the duration of his leadership by saying it is evidence that the party as a whole is unfit to govern.

It is an interesting question whether his greatest problems will come from his enemies or his friends. One of the heaviest burdens that has just descended on his shoulders is the huge weight of expectations of his followers. He is the canvas on which they have painted their dreams. The more naive of the Corbynistas are hailing his victory as “a democratic storm”. They talk as if Britain has just been swept by a revolution.

They have certainly overturned the barricades in the Labour party. But the Labour party is not Britain and Britain remains under the rule of a Tory government and will do so at least until 2020. This heady summer of Corbynmania will be followed by long, hard seasons of battle on the national stage against opponents rather more formidable than the internal ones that he trounced in the leadership contest.

While Corbyn celebrants cheered, David Blunkett wisely cautioned them: “Euphoria is easy.” In the self-validating echo chamber of a leadership rally or when communing with the like-minded on social media, it is possible to trap yourself into believing that everyone thinks exactly as you do. Now will come the rude discovery for some of his followers that they are not as representative of broader Britain as they might think. A quarter of a million votes looks like an impressive total on the scoreboard of a Labour leadership contest; it is a thin sliver of the national electorate. Lest anyone needs reminding, back in May, more than 11 million of them decided they wanted David Cameron back in Downing Street.

Since Labour’s defeat, a variety of post-mortems have been published by bodies such as the Fabians, Policy Network and the Smith Institute. None of them Tory front organisations; all of them Labour-sympathetic institutions. These interrogations of Labour’s loss have differences of emphasis, but all come to the same broad conclusion. Labour was rejected and came 98 seats behind the Tories because insufficient voters regarded Ed Miliband as a suitable occupant of Number 10 and too few trusted the party with tax, borrowing and spending.

Not one of these investigations into Labour’s defeat has come to the conclusion that the party failed because it was not left wing enough.

The contention of Corbynism is that all these careful studies are just wrong. The claim is that there is a route to power by recruiting voters from the Greens, from Nationalists and from among non-voters who have hitherto declined to turn out for Labour because the party has never offered them real socialism.

Well, now that thesis will be put to the test and the experiment starts soon. Big electoral challenges loom on the horizon: the May elections in Scotland, in Wales and for the mayoralty of London where Sadiq Khan, not a Corbynista, but endorsed by them, will be the Labour candidate.

Then there are the personal expectations that swirl around the new leader. Consider the case of John McDonnell, the MP for Hayes and Harlington, fellow veteran of the Bennite left in the tiny Socialist Campaign Group of MPs, and a close friend of the new leader. Without his efforts, Mr Corbyn might not have got into the contest in the first place.

Mr McDonnell has spent many unrewarding decades in the wilderness, many fruitless years tending the scarlet flame when his brand of politics was denounced and disdained by a succession of Labour leaders. He – and who can blame him? – strongly feels he ought to have a handsome slice of the spoils of this victory. He wants to be shadow chancellor. He won’t be a very happy man if his ambitions are disappointed.

His enemies are the more predictable challenge confronting Jeremy Corbyn. It is from his friends that he may ultimately face the most personally piercing slings and arrows. His support was big. That is not a guarantee that it will necessarily be enduringly solid. He won in part because these are volatile times. Some of those currently intoxicated by his victory are going to end up feeling terribly disappointed by him. For that is the fate of all leaders. Then will come the first accusations of treachery.

That won’t be a nice experience for a man who devoted his previous life to crying betrayal at all his predecessors as Labour leader.