David Cameron calls it “the sweetest of victories” – and, from his point of view, you can see why it is a sugar rush. The election result has made idiots of the pollsters and fools of those, myself included, who placed too much faith in their numbers. Like the climactic act of a Jacobean revenge drama, the political stage is littered with the corpses of his defeated rivals. His many internal enemies are, temporarily, silenced. Those hungry to snatch the crown – hello, Boris – must hide their ambitions for a decent period.

His majority would be called pathetic in other circumstances. It will look a lot less impressive and a lot more perilous when everyone sobers up. But it is better than his party has achieved for 23 years or anyone expected. Himself, if he were honest, included. It was only a few days ago that Team Cameron was war-gaming its hung parliament options. When the exit poll was broadcast at 10pm on Thursday, he was at home in the Oxfordshire village of Dean surrounded by his closest aides. Several in the room asked: “Can this be right?”

Over at Labour headquarters, stunned staffers reacted to the poll by placing their hands over their mouths, as if they had just witnessed a motorway pile-up. Which was what the night became for Labour. Another advantage for the Tory leader is that Labour will not be a functioning opposition for some time as it argues with itself about what went wrong. Many have drawn parallels with the shock of 1992. After that defeat, there was at least this consolation: the electorate had telegraphed a clear message about what Labour needed to do to get back into competition. Find an attractive communicator with the capacity to win on the centre ground. The result was Tony Blair and three election victories. Labour’s post-mortem this time will be more tortured because its defeat is open to several interpretations. The left will point to obliteration in Scotland, and elsewhere to the leakage of support to the Greens, as evidence that it was not left enough. Modernisers will say that their party will not see power again unless it rediscovers an ability to reach centrist, aspirational voters, a case made by Chuka Umunna, a contender for the leadership, in today’s Observer. Others will point to the support lost to Ukip, now threateningly positioned second in a lot of Labour seats, and say that the imperative is to re-connect with the “left behind”.

One of the paradoxes of this election – a horribly bitter one for the Lib Dems – is that the Tories achieved their majority by murdering the partner they governed with for the previous five years. When David Cameron stood on Downing Street paying tribute to Nick Clegg, the former Lib Dem leader surely laughed through his tears. He helped to insulate Mr Cameron from the right of the Tory party and shouldered a disproportionate degree of the blame for unpopular decisions. The Tory leader paid back the Lib Dem by ruthlessly targeting and devouring his seats. The Lib Dems thought they could tame the Tory wolf; they ended up in its stomach.

Talking to strategists and politicians across the parties, I find a broad consensus about what happened. Playing “the England card”, the tactic the Tories alighted on in some desperation, was effective in scaring voters south of the border with the thought that a Labour minority government would be the puppet of the Scottish Nationalists. One thing I did get right was to warn earlier in the year about the potential potency of this line of attack. Labour, Lib Dems and Ukip agree that it hurt them all as a significant segment of the English switched to the Conservatives to vote against the SNP.

When the pencils of swing voters hovered over the ballot papers, the fundamental rules of elections asserted themselves. No party has ever won when it is behind on both leadership and trust with the economy. Ed Miliband had a better campaign than many expected, but Milifandom was an illusion. He never came close to closing the deficit with David Cameron when voters were asked to say whom they preferred as prime minister. Labour always trailed badly when voters were asked the crucial question about who they most trusted with the economy.

Existential questions now face the vanquished. There is also a big one for the victor, the answer to which will determine his place in history and this country’s future. David Cameron will finally have to decide what sort of prime minister he wants to be. Is he a unifier or a divider? Will he behave like the sectional leader of a party for prosperous, and mainly southern, England or does the prime minister aspire to represent the whole country, including the many millions who did not vote for him? We have had several incarnations of him over the years as he has shape-shifted from the husky-hugging Cameron of his time in opposition to the divisive and negative Cameron of the election campaign. In the remarks he has made since his re-installation at Number 10, he has sought to mantle himself as a “One Nation” Tory, a label he used in the speech he gave at his constituency count and again in his remarks outside Number 10 on Friday lunchtime.

Tory moderates hope this means they have got their version of David Cameron back. We will see if they are right or set to be disappointed once again. Even if he is sincere, formidable obstacles stand in his way. First, and most obviously, we are clearly not One Nation. Never has this kingdom looked more disunited, a polarisation amplified by the electoral system. Southern England outside London looks like a one-party blue state. Scotland looks like a one-party nationalist state. The outcome of this election is a perfect storm for the SNP. They have reduced Labour to rubble in Scotland while the Tories govern from Westminster. Many Conservatives are coming round to the view that a federated union is the only hope of keeping the United Kingdom together. Even if David Cameron has it in him to make a big accommodation with Scotland, that may not be enough when the nationalists are now armed with so much opportunity to drive the two countries further apart.

I am interested in how the Tory leader thinks he is going to reconcile his aspirations to be a unifying prime minister with the many contentious things his government plans to do. George Osborne is re-ensconced at the Treasury having decided that his ambitions for the succession are better served staying in the financial wheelhouse. There are more swingeing spending cuts to come and an unitemised £12bn reduction in benefits for the working poor. The chancellor also has to find some £20bn to meet all the tax and spending pledges the Tories recklessly sprayed around during the campaign.

And even if David Cameron genuinely desires to be a consensual leader, will his party let him? Revolting Tory backbenchers made the last parliament the most rebellious since 1945. Then he was buttressed by the Lib Dems. The coalition had a majority of more than 70. This result may have given the Tories more seats, but at the same time it has left the prime minister much weaker in parliament. He has a wafer-thin majority and no Lib Dem buffer between him and his backbench irreconcilables. He may come to look back on his days with Nick Clegg as bliss. The country may eventually develop more appreciation than it expressed at the election for how the Lib Dems restrained the Tories.

I stood in Downing Street in 1992 when John Major returned there after his surprise victory. Pundits and pollsters were gobsmacked. Tories were euphoric. The opposition was in wrist-slitting despair. Sir John wanted to be a One Nation prime minister, sincerely I think. He spoke of “a nation at ease with itself”. The nation ended up united in its loathing for his fratricidal government. By the autumn after his victory, the public was already regretting its choice as the Tories began to war with each other over their ideological trajectory in general and Europe in particular.

Europe will again be the issue that convulses this Tory government and with even more ferocity than in Sir John’s day. This result does not erase the fundamental problem that faces Mr Cameron when he attempts to embark on his renegotiation of the terms of British membership. What might be an acceptable outcome to our European partners will never satisfy a big faction of his party.

This government will not be popular for long. In fact, the Tories were not popular on polling day: 63% voted for someone else. The Tories are not liked, even by quite a lot of those who voted for them. Many did so only because they fancied the alternative even less. His fragile majority will be acutely vulnerable to rebellions, ambushes and blackmail by a handful or two of backbenchers. That will get worse when the majority is eroded as byelection losses take their toll. By announcing that he has fought his last general election, he has put a sell-by-date on his premiership.

A vanishing majority, dissipating authority, a lot of cuts to come and expensive promises to keep, a fractured kingdom and an EU referendum that will split the Conservative party asunder. David Cameron should savour his “sweet” victory while he can. History tells us that it will turn sour.