Unity may have been the watchword of Keir Starmer’s campaign for the Labour leadership, but change is the word really worth thinking about if one wishes to understand the man who will almost certainly succeed Jeremy Corbyn on Saturday.

That might seem counterintuitive. Over the past four months, the Shadow Brexit Secretary – who I have interviewed for the upcoming NS Spring Special – has sought to convince Labour members that his leadership need not mean drastic change, at least when it comes to policy. When he has explicitly called for change, Starmer has tended to express it in terms of ends rather than means. Hence the emphasis on party unity. It is a difficult message to disagree with, especially if one is told that the values and policy platform Labour is uniting behind won’t change.

Plenty of people on the Labour left have been convinced by that message. Yet for others, namely the Corbynite unions and other supporters of Rebecca Long-Bailey, the conflict with what Starmer is allowing to go unsaid is too great for the message to wash. Or, to put it more simply, they believe that the changes implied by Starmer’s pitch will be followed by the changes he insists won’t happen. They suspect an end to factionalism means kneecapping the left in Parliament, at Labour HQ and at the grassroots, and that uniting to win means splitting the difference with Corbyn’s internal opponents on policy.

Yet that is precisely the change that most of the 88 MPs who backed Starmer for the leadership want. Will they get it? Labour will certainly get change before it gets unity. Change, and a desire to effect it, has always been the locomotive force of Starmer’s politics. It sounds like a truism: all politicians want to change things. But Keir Starmer’s life has been spent changing institutions so that they might be better vehicles for progressive values, as he defines them. On that much, he has been consistent since boyhood.

Starmer the lawyer and Starmer the politician are linked by a desire to change institutions – not in their aims, but in how they pursue them. Starmer was a teenage Labour activist whose real awakening came in a rarefied corner of the extra-parliamentary left. As a young barrister in 1990, he was among the radical advocates who left the Inns of Court to set up shop on Doughty Street. It is now a fashionable set, but it was then a redoubt for those who wanted to do what other barristers wouldn’t, or couldn’t – namely, human rights cases. There Starmer fought for trade unions, and against the Thatcherite state and the institutions that enforced its will, most notably the police.

The point of his practice, and those of his Doughty Street colleagues, was to pursue a rearguard action against the state. Parliamentary change was then off-limits to the left. Only via the courts could they win enduring victories, through muzzling institutions such as the police or by loosening the legal constraints on unions. He channelled his internationalism in the same way, fighting death-penalty cases in the Caribbean. Eventually he came to change the state from the inside, first as a human rights adviser to the Northern Ireland Policing Board, the body set up to ensure the PSNI discharged its duties to both communities fairly and proportionately, and then as Director of Public Prosecutions. It was there, he says, that he came to understand that institutions could, by evolving, change themselves.

He always hoped to change the Labour Party, too. Soon he will have his chance. Any number of labels have been applied to Starmer over the course of the leadership campaign – continuity Miliband, closet Blairite, soft left – but none really capture his politics. To do so, one must look beyond the factional traditions of the Labour Party and to the continent.

The QC Gavin Millar described his friend to me as an old-school European red-green. When I put that description to Starmer ahead of a campaign event in York earlier this month, he did not dissent. His ambition then was to see the Labour Party broaden its base to represent the liberation movements that had emerged as political constituencies in their own right over the course of the 1970s and 1980s: feminism; environmentalism; LGBT rights. Those politics, he says, have not changed.

But whether Starmer truly believes in nationalised utilities, a Green New Deal or the other policies from the 2017 and 2019 manifestos – and in most cases, he does – is less important to the constituent parts of the Labour Party than how he thinks it must change to put those values into practice. And it is practice that will matter, at least at first, much more than principle. That’s why briefings to the Sunday Times of wholesale institutional change, the sackings of key Corbyn aides and a clearout of the Shadow Cabinet are overwhelmingly likely to be borne out in the first days of a Starmer leadership.

Opening one’s tenure with what some on the left have not unreasonably described as a purge might look like a funny kind of unity gesture. But the Labour Party is only the latest – and the most challenged – of a series of institutions Starmer has seen as constrained by their own culture. His calculation is that changing it will not be painless. The gamble – and it is a big one – is whether he can effect this change without plunging his party even deeper into the factional mire.