Politicians and journalists rarely make good friends, but David Davis and I got on pretty well in the period we campaigned on civil liberties together during the last Labour government. We lunched, he came to my home for dinner, I introduced him to friends in New York and we often shared the same platform, notably in the Guardian debate at the Hay festival, when he and I took on Charles Clarke and the legal expert Conor Gearty on the motion: “Does the left still care about liberty?”

I came to like him a lot and to admire his bounce and pugnacity. Among all the politicians I knew – with the possible exception of Dominic Grieve – David possessed the deepest instinct for liberty. He seemed to me to be the archetypal English democrat who possessed a great reverence for the rule of law as well as an equal suspicion of the accumulating powers of the state. He spoke brilliantly at events I was involved in staging, often preparing a long and complicated address with a few notes, just moments before he went on stage. He was bright, energetic and, for that period, liberty’s hero.

Today, with David now at the heart of government as the secretary of state for exiting the EU, this all seems a very long time ago. We are now on opposite sides and I recognise almost nothing in the politics of the man who stood at the Convention on Modern Liberty in 2009 and asked: “What is the point of Britain if it does not adhere to the freedoms that made it? What is the point of parliament if it does not uphold its most sacred trust as a guardian of our liberty? What is the point of government if its principles aim to maximise fear and minimise our freedoms?”

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During those “liberty years”, he always insisted that parliament was the bulwark against authoritarian government and the ultimate protection of the British people. Yet, within just a few months of accepting the job of Brexit secretary from Theresa May, he insisted that the executive must deprive MPs of the right to debate the triggering of article 50, a move that in the words of Lord Pannick, who led Gina Miller’s challenge against the government on this issue, undermined parliament and “deprived people of their statutory rights”.

This jettisoning of his principles didn’t surprise me a great deal. Just after becoming Brexit secretary, he removed his name from the legal challenge to the data retention and investigatory powers bill, which forces internet companies to keep records of people’s searches and allows authorities to hack devices secretly. These are powers that he would not have contemplated in his former life, but he acceded to them without demur.

As we know, the issue of Brexit is so viscerally potent that it can damage personalities and relationships forever. It is obvious that for David nothing matters as much as leaving the EU. In the first six months of his tenure, he dumped, apparently without qualm, two key parts of his political character – his respect for parliamentary sovereignty and his love of individual liberty. I won’t rehash the arguments about state power here, but if you hold a belief that indiscriminate retention of data by the state is unjustifiable, that view cannot be easily changed without casting doubt on the authenticity of your original conviction.

Maybe he used the stage of liberty to occupy his energies during the voluntary wilderness of the Cameron years

Perhaps he never really believed it in the first place. Maybe this strikingly extrovert political character simply used the stage of liberty to occupy his energies during the voluntary wilderness of the Cameron years. We have all been changed by the referendum and, on the whole, I opt for an interpretation that sees Brexit as a psychologically transformative event that has skewed the values and beliefs of hundreds of thousands of people. David may be just one of many.

But this is a sympathetic reading and it is not supported by his attitude to parliament over the last 18 months, especially his promotion of Henry VIII powers – rule by ministerial decree without debate in the the House of Commons – which he condemned so frequently in my hearing. For a man who so often championed parliament’s power to hold ministers to account, he has, since becoming a minister, been by turns truculent, casual and almost contemptuous when questioned by MPs about the negotiations with the EU. The key moment was on 6 December when he finally admitted to Hilary Benn, the chair of the Commons Brexit committee, that the 50-60 impact assessments he promised had been carried out did not exist. This is what he said in June: “In my job, I don’t think out loud and I don’t make guesses. I try and make decisions. You make those based on the data. That data is being gathered. We’ve got 50, nearly 60, sectoral analyses already done.”

In fact, no data had been gathered on Britain’s economic future outside the EU, apart from documents hurriedly cobbled together and then shrouded in risible secrecy to cover up the lie. In his former life, David would have seized on this ministerial dereliction and wept tears of anger at the incompetence and hypocrisy of it all, particularly as more thorough impact statements, carried out by the mayor of London, Sadiq Khan, convincingly paint a picture of lost jobs, growth and investment, post Brexit.

A few days later, he told Nick Ferrari of LBC he didn’t have to be very clever or knowledgeable to do his job – just calm. He is wrong. In the second round of negotiations, he has to be very clever and he must master an untold number of briefs on different aspects of trade. On the evidence of the leaked letter that he sent to the prime minister in which he complained the EU was already penalising British interests in the case of a no-deal Brexit, it might seem that he’s not really up for the challenges of the next few months. It was all right for him and the prime minister to talk tough, but the moment the EU reciprocated he reacted with a whiney tone that did not seem like David and does not bode well for the negotiations or the inevitable problems that will come with the unravelling of the “agreement” on the Irish border.

Watching my former debating partner over the last year and half, I find I have lost no affection for him. I see his energy, the fighting spirit and his good-humoured resilience, yet I am struck by a couple of things. While I would be happy to be in the jungle or stuck in a lift with him, I would draw the line at being stranded in a planning room with him. The second thought is about Brexit’s destructive power on relationships and principles. Whatever happens in the next round of negotiations or, indeed, with the final deal, this damage to our faith in each other is going to take a very long time to repair.