Can you forgive him? That depends on whether you think Nick Clegg venal or just a political idiot. Seeking power was no sin, as that’s the purpose of politics: he is to be judged by how he used it. In a rich crop of self-justifying politicians’ books this autumn, Clegg’s Politics: Between the Extremes is first to invite an assessment of how he did.

He might have been wiser to keep his head down and hope the country has a short memory. But that would be out of character, as his own account reminds us of one grave political misjudgment after another. Almost everything he colluded with in the Cameron government was an error, while almost all he achieved was piffling in comparison. His role now is as a warning beacon of what not to do in future coalitions.

How Icarus-like was his fall from the dizzy days of Cleggmania. Back in 2010 “I agree with Nick” was mirrored by opinion polling. For the record, this paper backed the Lib Dems in 2010, though its political columnists – myself included – wrote supporting Labour.

It took decades of local pavement-pounding for the Liberals to grow from a taxi-full of MPs into a 57-strong coach-load at Westminster. But after Clegg led them to a thrashing last year, they are now back in an eight-MP people carrier (with probably only four MPs after boundary changes) – and have few local councillors. They have deserved it.

Start back in that 2010 sunlit No 10 rose garden, with toe-curling smiles that look yet worse in retrospect. Clegg and his party had to make one big call: was the country indeed on the verge of Greek-style bankruptcy and in need of David Cameron and George Osborne’s emetic austerity medicine? They fell for the bait and called it wrong, as even the likes of Mervyn King, Bank of England governor at the time, now agree.

‘Toe-curling smiles’: David Cameron and Nick Clegg – the then prime minister and deputy prime minister – hold their first press conference in the rose garden of No 10. Photograph: Christopher Furlong/PA

The Lib Dems swallowed the story that the country needed a boiling down of every function of the state to its bare bones. They were useful idiots for what was always an ideological project: Cameron and Osborne said within a short time that even once the deficit was down, there would be no restoring of cut-down public services.

In accepting extreme cuts as an economic remedy, Clegg abandoned his party’s greatest thinker, Keynes, who would have gone for growth through government investment. But even if Clegg has been right on austerity, why did he let the axe fall on the most vulnerable? In policy after policy, the bottom half bore the brunt as VAT rose, while the top had an income tax cut and kept their many benefits: tax reliefs, and cuts to capital gains and corporation tax.

The rightwing thinktank, the TaxPayers’ Alliance, showed how Osborne raised the lifetime tax-take from the bottom 20%, while tax for the top 20% fell. Even Clegg’s flagship raising of the income tax threshold was soon revealed by the Resolution Foundation to direct most of its huge £10bn cost to the benefit of the upper half while low earners gained little.

Guardian readers need no reminding of the extreme severity of the benefit cuts – hitting children, mothers and people with disabilities hardest. Clegg had no need to fall in with the vindictive spirit of the bedroom tax, the sale of swaths more social housing, or the ever-tightening screw of work capability tests, as food banks opened everywhere. Labour supporters seeing the Lib Dems sitting on their hands and saying nothing may never forget. But then social justice was never the Lib Dems’ strong point: good on human rights and the environment, social equality is not in their DNA.

In 2012 Andrew Lansley’s health and social care bill was stumbling in parliament, as it dawned on the Lords that this was a project to blow the NHS into fragments by setting up a market tendering every shard out to any bidder. “No top-down NHS reorganisation,” said the coalition agreement, and the Lib Dem grass roots rebelled. But they were soothed by Shirley Williams, and Clegg voted it through.

Why? Cameron was astonished not to lose the bill. So disastrously unworkable has it proved that it’s now being dismantled again, but deep, lasting damage was done to the NHS that Clegg could easily have stopped.

The charge list is long. The value of his pupil premium remains disputed, and he has suffered enough for his tuition fees debacle. He boasts of pushing through 5p plastic bag charges, but he should have stopped the banning of land-based wind turbines. He was treated as a minor irritant in the great Tory project – and that’s all he was.

In his book, Clegg observes Tory savagery on benefits and housing as if he were a non-participant; yet he had the power to stop or soften much of this. What makes him more of an idiot than a villain is his weak understanding of the power he had.

Take his failure to seize the one great prize his party sought for decades – proportional representation that would make such coalitions a fixture. His party would justify a pact with the devil to secure electoral reform. But out of sheer incompetence, Clegg blew that chance. He got his referendum, but only on AV: the weak “alternative vote” even his own side didn’t want. Fatally, he failed to force Cameron to pledge his party’s support. The same team of Tory liars who swung the Brexit vote ran the anti-AV campaign, preposterously claiming the tiny cost would deprive babies of NHS incubators and our boys in Afghanistan of kit. How Cameron’s crew laughed when Clegg lost.

Had he secured PR, the political landscape would be changed for the better, beyond all recognition. The Tory party would have split between pro- and anti-EU wings. Labour would split between Corbynites and social democrats. Ukip and the Greens would have their fair share of seats.

Above all, citizens could vote for a party closest to their views. At a time of turmoil and alienation, it’s never been more urgent to restore a closer link between what people want and what they get in Westminster. I can never forgive Clegg for bungling that once-in-a-generation mission so badly.

History may remember him as no worse than clueless. But one tragedy is that the country needs a strong unequivocal pro-European party, as the Lib Dems once were. Clegg’s miscalculation of everything has left behind only a tiny rump with too little heft to influence the battle ahead.