Bravo, Andrew Adonis: the final transformation of this mild-mannered policy wonk into battling anti-Brexiter is a fitting way to end a year no less defiant of prophecy than 2016. And rather a cheering one, as it happens.

In truth, the former Labour transport secretary and No 10 policy chief has been steadily morphing from bookworm to kick-ass over the last 18 months, his interventions growing more trenchant, his cerebral manner increasingly matched by a taste for action.

Adonis on Brexit: ‘No mandarin backs May. Government has broken down’ Read more

Yet his resignation as chair of the national infrastructure commission on Friday captured this evolutionary process in a single, splendid moment of political drama. In his letter to Theresa May, Adonis characterised Brexit as a “populist and nationalist spasm worthy of Donald Trump”, described the European Union withdrawal bill as “the worst legislation of my lifetime”, and accused the prime minister of “allying with Ukip and the Tory hard right”.

The list of beefs is plentiful: in his interview with the Observer, he called for Chris Grayling, the transport secretary, to be sacked over the bailing out of Stagecoach and Virgin, presently contracted to run the East Coast line until 2023. He has been, as Rik Mayall would have put it, “pretty fierce” about avaricious university vice-chancellors, the “Frankenstein’s monster” of tuition fees, Birmingham city council, the providers of 4G phone coverage, and much else besides.

But it is Brexit that has energised him and driven his public metamorphosis from the owlish curator of Roy Jenkins’s legacy to an energetic activist campaigning for all he is worth to prevent Britain’s departure from the EU. As he put it on BBC Radio 4’s Today programme, this means “arguing passionately with the British people as to why staying in the EU is the right thing to do”.

There are those who say that this is an intrinsically improper project. A referendum was held, a result declared, and an argument concluded. How dare “remoaners” continue to voice their opinions on a settled matter?

To which I am tempted to respond in words of one syllable – but will not, in deference to the festive spirit. Still, it is absurd to suggest that the vote on 23 June 2016 was the final and definitive statement on the UK’s membership of the EU. It is the mark of a free society that debate continues, that dissent is not only tolerated but embedded in political discourse, and that political decisions can be reversed.

The overwhelming probability remains that Brexit will indeed go ahead on 29 March 2019 – as Adonis knows perfectly well. All the more reason, then, for those who regard this as a disastrous course to urge upon the electorate a change of trajectory.

For his part, Adonis calculates that the folly of leaving the EU will become ever more clear in the coming months, and that Labour’s leadership may yet replace its five-sizes-fit-all position with one closer to his own. This persuasive enterprise may well be doomed – but so what? Do we really want our politicians to be a tribe of bobble-heads who long for nothing but a quiet life? True patriotism is often to be found in the courage to challenge majority opinion.

It does Adonis no favours, of course, that he hails from what is presently the least fashionable ideological postcode: the blasted heath of liberal centrism. This battered position has few, if any, defenders in the upper reaches of May’s government; and the founding principle of Corbynism is that the Blair-Cameron era was a period of unmitigated national disaster.

To declare oneself a centrist these days is to invite fairly arbitrary denunciation as a “neoliberal” (or “neoconservative”, take your pick), metropolitan elitist, Old Etonian, Old Mandelsonian, friend of dictators, ally of the banks, relentless privatiser, fox murderer, war criminal … and that’s just the vanilla stuff. Naturally, Adonis is an untouchable because of his close association with Tony Blair and his membership of the House of Lords. Never mind that he is the son of a Cypriot immigrant, and was placed in care until the age of 11: he ended up in the wrong gang, didn’t he?

So it is easy for both Brexiters and the left to sneer at Adonis as an irrelevance from the past, beating his chest in desperation. Easy, but wrong.

For the unfashionable centrism that he incarnates is not defunct. True, it has taken a pasting, as the pathologies of globalisation have become ever more clear and governments around the world have failed to keep pace with the social cost. In consequence, technocrats have been bulldozed by populists.

The liberal fear is that identity politics will ignite a spitting cauldron of Trumps and Brexits

But much more survives of the liberal inheritance than is commonly supposed. At its root remains a still-precious belief in pluralism, and the conviction that a decent society thrives on both diversity and an agreed core of common principles. It prefers debate, free speech and ceaseless social interaction to safe spaces, no-platforming and digital cantonisation. Its more thoughtful exponents fret that a polarised politics based principally upon identity groupings – of left and right – will never truly favour the vulnerable, the dispossessed and the disenfranchised.

Rules-based systems are often tiresome, but they are preferable to unbridled political tribalism. And as the tide of populism spreads across Europe, the liberal fear is that identity politics will ignite a spitting cauldron of Trumps and Brexits.

The Europhobic right will never be reconciled to what Adonis stands for, or what he proposes to do. But those on the left who deplore him should pause for thought. Progressive politics rarely follows a script. Who, for instance, would have predicted that gay marriage would be legalised by a coalition of Tories and Liberal Democrats? Since the June election, the Labour leader and his acolytes have been quite clear that the triumph of the left and its arrival in government is now only a matter of dates.

Which it may well be. But history is not obedient. It squirms, wriggles and flounces when taken for granted. So – my friendly advice for 2018 – don’t assume that the old alliance of left and centre-ground can be consigned for ever to the ideological dustbin, or confuse the exhilaration of political purity for wisdom. Don’t assume, either, that the next election is won just because the last went well. Don’t, in other words, sneer so readily at Andrew Adonis: you never know when you may need him. Happy new year.

• Matthew d’Ancona is a Guardian columnist