Wrong, wrong and wrong again. Was ever there a more crassly inept politician than Jeremy Corbyn, whose every impulse is to make the wrong call on everything? It’s not excitingly flamboyant red radicalism that has done for Labour, but his sluggish incompetence at the absolute basics of leadership.

How rarely he has had the chance to wield any power, but on Wednesday he had the very real authority to stop certain calamity for his party and call out Theresa May’s game-playing chicanery. The mother of all bombs is about to drop on Labour, but what does he do? He says: “I welcome the prime minister’s decision to give the British people the chance to vote for a government that will put the interests of the majority first.” What?

The Fixed-term Parliaments Act was designed to stop prime ministers dashing opportunistically to the polls when momentarily at the peak of their popularity. May can only gain two-thirds majority in the Commons if Labour agrees to its own annihilation – which he welcomes. Will this be the last disastrous disservice he does to his party?

Who knows what other clumsy damage he may inflict during the campaign. And afterwards, he might not go. Remember Tony Benn celebrating the millions of votes for “socialism” under Michael Foot’s 1983 political suicide manifesto, though Labour had crashed to epic defeat? Fantasy politics reign again when Momentum responds to May’s announcement by tweeting about the “path to victory for Labour”.

Among a few of the three-quarters of Labour MPs – 172 – who voted no confidence in Corbyn last year, I hear strange sounds. Thank God! This will put them out of their misery, as if shooting the sick dog Labour party would put it out of its misery. This election could purge the party’s mortal disease, like a toxic dose of mercury. Never mind if the cure is worse than the illness, at least it ends this time of total paralysis. But that’s another kind of fantasy politics.

Since last September’s failed challenge to Corbyn’s leadership, most of Labour’s senior people languish, uselessly immobilised, while a largely insignificant shadow cabinet makes no impact – Keir Starmer the one exception. Another putsch was planned by some MPs for the autumn out of desperation – even they fared no better than last time.

Waiting for the 60% of Labour party members who voted for Corbyn to come to their senses looked like a very long wait indeed. Talk of a new party to break away from the suicide party was heard only from those long out of parliament and out of touch. The fate of the SDP should be salutary warning to anyone trying to break the wicked first past the post stranglehold on all political innovation.

The prospect of losing scores of seats should temper any self-harming glee among Labour’s anti-Corbynites. Electoral Calculus predicts Labour could lose around 50 seats. Alan Johnson’s flight is a bitter blow, following that of Tom Blenkinsop, MP for Middlesbrough South and East Cleveland, who would have been defending a thin majority in a heavily pro-Brexit area. Labour’s fate will be well and truly sealed if other non-Corbynites jump ship: their duty is to stand and fight.

Labour’s manifesto may not contain much all MPs couldn’t campaign on: Corbyn has been no firebrand after all. His recent cascade of minor policies are fine in themselves: £10 a week more for carers by abolishing inheritance tax cuts, a £10 minimum wage, free school meals paid for by private school VAT, obliging companies to publish their tax returns, and more. But elections are rarely about policies. Elections are won by the best-led party; YouGov finds just 15% put Corbyn as best leader, against May’s walloping 49%.

Just watch those reasonable policies twist in the wind under Lynton Crosby’s fiendish hand: Labour will snatch your inheritances, kill parents’ private school aspirations, punish business with compulsory unionisation, and so on. Labour can only make radical policies fly when floated by a trusted leadership. Nothing Corbyn proposes is as radical as Blair and Brown’s £5bn windfall snatched from privatised utilities – but by then they had earned economic credibility the hard way. If you are near Manchester, visit the People’s History Museum’s timely exhibition on Labour’s 1997 winning election campaign to see how it was done. Corbyn may offer the casebook study in how to lose.

This election will be pure Brexit, up to the blood and guts hilt. One paradox is May’s belief it will set her free to rule with an estimated 126 majority; it will be her curse. Not captain of her party, but its captive, she will be in thrall to a virulent rightwing rock-hard set of new Brexit MPs, nasty party through and through.

A Brexit election has Tim Farron crowing with relish, the Lib Dem’s crystal clear pro-EU stance destined, thinks Crosby, to regain 27 seats lost to Tories in 2015. Brexit and Labour? Corbyn mumbling into his beard, conflicted.

The 48% confront the prospect of May, imperial and imperious demanding “unity” where there is none, badging all opposition as unpatriotic in the face of Brexit negotiations. All they can do is choose whichever anti-May candidate is best placed to deny her a monstrous majority lasting all the way to 2022. With no further say on whatever deal May retrieves from Brussels, politics has rarely looked grimmer.