A message to all those who didn’t back Jeremy Corbyn – that’s most Labour MPs, a small majority of paid-up members, the Guardian and myself: today there is only one direction of travel. No going back, no alternative, no possible re-run. This new party’s swelling ranks want no more of the old politics, no more caution and obfuscation, no more talking tough while sneaking in good by stealth.

The new shadow cabinet of the willing sets off on a journey into terra incognita. At least at the helm there is the unity of purpose of a leader and shadow chancellor in political lockstep. But among the others some are signed up for the Corbyn trip, others are only joining to try to steer the party away from the rocks. The lack of women in top jobs is a damaging handicap, after years of Harriet Harman’s work at making Labour best for women. Behind the new team sits a sea of funereal-faced MPs, the appalled and the merely pessimistic.

Those former shadow ministers who have stomped off in a huff and a flounce make a serious error by deserting their posts. If they fear contamination with what they expect to be the Corbyn car-crash, they will find themselves blamed for failing to stay. Backbenchers who snark and snipe, helpful only to the enemy, can expect no thanks from any side of the party. If Corbyn fails, they risk being seen as part of the reason why.

Flip-floppers and ditherers nakedly calculating the odds on the least career-damaging course will find themselves left stranded, whatever happens next. Because whatever follows will not be any return to their past but something else again, without them. They preach being serious about power and yet abdicate before waiting to see what policies they can help shape.

The only way to travel is in hope – in politics as in life. I hoped for Gordon Brown’s success, I hoped for Ed Miliband’s. What’s the point of dread and despair? Corbyn has shown he can enthuse multitudes with optimism, sincerity and straight-talking. In his acceptance speech he called for “working together to achieve great victories, not just electorally for Labour, but emotionally for the whole of our society to show we don’t have to be unequal. It doesn’t have to be unfair, poverty isn’t inevitable, things can, and they will, change.” That’s essentially what Labour people feel and always have, but that sound of radicalism is usually stifled for fear of frightening the voters.

The awful truth is that too many Labour voters have already fled in all directions – to Ukip, to the SNP, to Tories in the south, with the West Midlands no longer secure. There is no safe harbour for Labour now. Looking for sweet spots with policies for niche voters has failed. Facing both ways, by sounding austerian while secretly planning Keynsianism, pleased few. But then the Tory message did none so well either, with a mere 12-seat majority, despite crafty bribery of select demographics, despite a Labour near collapse.

What’s the point of despair? Corbyn has shown he can enthuse multitudes with optimism, sincerity and straight-talking

No one has the answer to widespread disaffection with mainstream politics. So the great Corbyn hope-and-change message is worth trying. Wipe the grim looks off doubting faces and try sheer conviction. Besides, with his 60% mandate, there is no alternative for Labour now.

As I write, experimenting with hope that Jez can, I can feel myself shoving into a cupboard all the reasons to fear that he can’t. Here are the bulky items to be bundled in, with the door propped shut to stop them bursting out.

Start with the damage done by the monstrous megaphones of the Tory press. Even Ed Miliband had a polite pause before the assault, but now they are more crassly brutal than ever: “Red and Buried” (Daily Mail); “Bye, Bye Labour” (Express); “Leader Nightmare” (Sun); “Death of Labour” (Telegraph). And much more. Perhaps monstering earns underdog sympathy, with contempt for the press as rife as contempt for conventional politics.

Corbyn supporter Owen Jones takes us behind the scenes as Jeremy Corbyn is elected Labour party leader. Link to video

Next, try to ignore the hailstorm of crude messages blasted from David Cameron’s multimedia machine. The single mantra from every Tory minister says the Labour party is “a threat to our national security, our economic security and your family’s security”. Does the sheer firepower of endless repetition work? As this idiocy ricochets around the web, it may yet rebound for insulting voters’ intelligence.

Next, try to push into the cupboard the enormous electoral obstacle: Labour needs to win 94 English and Welsh seats. For that, four out of five of the requisite extra Labour votes need to be stolen from the Tories. Even summoning the young, the poor and the alienated will not suffice because most of those available votes are in seats already Labour-owned.

For now, shut that cupboard full of doubt and instead hope the Corbyn magic can reach beyond already left-leaning citizens. All round Europe there have been political earthquakes in a volatile anti-politics age: the surprise is that Britain’s scratchy, irascible electorate hasn’t expressed its underlying anger that ordinary people paid the price for the bankers’ crash. But Labour’s great danger now is swallowing itself up in civil strife over who is moderate and who extreme, with threats of deselections on one side and secret venomous cabals on the other. If shadow foreign secretary Hilary Benn and deputy leader Tom Watson’s firm stand on the EU and Nato hold, then unity is feasible.

The real extremism is across the House of Commons in the most radical government of our lifetime – and it needs a forceful opposition to show voters that’s where the real danger to the country lies. Out there, the Met Office says climate change has just reached a fateful turning point, yet Cameron drops the “green crap”. Out there, he ignores Europe’s refugee crisis. As the trade union bill crushes already feeble employee rights, more wealth will be drained from wages into profits.

This week’s welfare bill cuts £750 from “hard-working families’” tax credits, while the spending review brings another £20bn harrowing of public services. The great question for Corbyn is whether he and his team can muster the persuasive power to convince voters that deficit reduction by shredding the public realm is a needless, political state-shrinking project.

The effrontery of Cameron’s speech last Friday, opening public services to more privatising, suggests they are recklessly off the leash. When they overreach, they can be beaten. But if Corbyn fails, the Labour party membership will not reward any who wrecked his chance to try.