It’s all over: hens’ teeth are commoner than senior Labour people who think otherwise. Candidates soldier on, but Labour folk of left and right pinch themselves to see if they might wake up from this dream/nightmare. As certain as elections ever can be, in just over two weeks Jeremy Corbyn will be Labour leader. A line is drawn for ever under the previous Labour era, and a new party has been created in a thunderflash. What then?

This late-summer delirium comes after a year of electioneering, with people wearied by hollow promises, empty jargon, callow attacks and artificial slogans. Corbynmania wafts in on the anti-politics breeze. Here’s a normal-looking 66-year-old who sounds as if he means what he says, speaks without a sock in his mouth, unconflicted by compromise, caution or circumspection.

Exhilarating novelty, the joy of upending the establishment and thumbing a nose at conventional expectations; by picking an unknown outsider, Labour may get a bit of an autumn lift in the polls. He made front-page FT news yesterday with his assault on “ludicrous” corporate pay, attacking Rupert Murdoch’s empire and telling bankers to wake up to Britain’s “gross inequalities”. Who disagrees with all of that? But if being right was all it took to win elections, the Tories wouldn’t be in power.

What next? The rightwing press has hardly sharpened its pencils for the assault to come. For a foretaste, sample the Mail on Sunday’s fantasy spread on “Prime Minister Corbyn and the 1,000 days that destroyed Britain.” (The flight of the rich and Premier League footballers, and the collapse of the property and stock market ends with UN peacekeepers rescuing the nation from the flames.)

Monstering may earn him underdog sympathy. At prime minister’s questions, Cameron lacks the elan to stand aloof: his red-faced bullying jibes may bounce off Corbyn’s Teflon authenticity.

And then what? The Labour party is riven. With only 20 MPs nominating and voting for him, the rest risk suddenly seeming yesterday’s people of the old dead party. How those Labour MPs behave will determine the party’s future, almost as much as how Corbyn runs his leadership. The auguries are not good, as a shuddering split divides old colleagues throughout the party. Wherever two Labour people gather together, the fault line shakes old friendships. War, peace or truce, Labour’s future hangs in the balance if both sides forget how unelectable is a split party obsessed with itself: ask John Major.

This doesn’t feel like the schism of 1981 when the SDP split away. The iron fist of the electoral system will prevent anyone in their right mind trying that again. While Corbyn’s aggressive tendency spits out tweets calling people like me Red Tories, all depends on the tone he strikes. Will he sanction wholesale deselection of MPs in the coming boundary-change turmoil? Hotheads on the kneejerk right can be as dangerous, with talk of secret cabals, instant revolt and legal challenges. They need to understand and accept this revolution.

Whatever emerges next, there will be no return to the old party. Will Corbyn listen to a wide spectrum of MPs in his shadow cabinet? If he can’t compromise with his own party, he will never compromise with the voters.

He has electrified people revolted by the cynical politics of winning by a little bribery to niche voters in marginal seats. But so far he has galvanised only natural left-leaners. His campaigners say Labour can win if poor and young non-voters can be enthused to the polls. If only that were true; but there just aren’t enough of them. In The Mountain to Climb, Fabian research pitilessly strips bare the harsh electoral facts. Labour needs 94 more English and Welsh seats. To win those, four out of five of the requisite new Labour voters need to be stolen from the Tories. Perhaps Corbyn is a game-changer, but without Tory votes he needs to be a magician.

Even if he caused a turnout surge as strong as the SNP’s in those marginals – 7.4% – and even if all of them voted Labour, that only yields 52 seats. Even if he won every Lib Dem and Green voter, that’s not enough. If Ukip collapses, their vote divides equally between Labour and Tory, so no use. The old out-vote the young, so can he win more over-65s? Can he summon the left without leaking centre-left voters to the right?

Read this research and tear your hair out with frustration – but you can’t block your ears. Our shameful electoral system denies the right to vote for a clear leftwing party with any hope of holding power. Proportional representation would let a leftist party and a social-democratic party each attract their own voters, combining in coalition. But without a fair voting system, Labour can only win power by reaching some voters who went Tory last time.

The idea of Corbyn winning Tory votes stretches credulity. I remain convinced that Yvette Cooper has the best chance of holding the party together, appealing widely – especially to women – as a tough anti-austerity economist holding the Tories’ feet to the fire. If Labour can’t elect a winner, nothing will stop the Tory evisceration of public services and the welfare state.

The next question is what happens if it becomes plain that Corbyn is failing and Labour’s fortunes remain dismal, or sinking yet lower. Gordon Brown and Ed Miliband both dragged down their party: while Cameron was more popular than his party, they were considerably less popular than theirs, yet they clung on.

On Any Questions last week I asked Corbyn what I would ask each candidate: if he failed, would he fall on his sword and step down? He fudged. He said, “There is a procedure that an election can be triggered on an annual basis if people really want to do that.” I asked again but he shrugged it off with, “We’re crossing an awful lot of bridges which we haven’t reached yet.” So he does do conventional political dodging too.

Of course, he could be forced into another leadership election by the majority of MPs who oppose him – but ferocious warfare would break out right through the party causing immeasurable harm. It baffles me that failing leaders never do the honorable thing, even when it’s plain they are slamming their party into an electoral brick wall.

Some senior Labour party people said that promising to stand down might boost Corbyn further: they say some party members like the idea of a bit of radicalism for an interregnum, followed by a more plausible prime ministerial contender in time for 2020. If that’s what some Corbyn supporters think, that’s delusional. Within two years we will face the EU referendum, when Labour’s policy needs to be crystal clear and unequivocally for staying in Europe.

It’s the Tories who are already shaping up to fall apart over their old EU blood feud, with eight cabinet ministers reported to be considering joining the No side. What a tragedy if Labour’s own state of disorder, not least over Corbyn’s EU prevarications, ends up assisting a vote to eject us from Europe. What’s at stake goes far beyond Labour’s own fortunes.