The “ABC” campaign to keep us in the European Union, led by Adonis, Blair and Cable, is feeling its way towards the next big battle.

As I noted here last week, Andrew Adonis in his letter of resignation from the National Infrastructure Commission seemed to accept that we will actually leave the EU, and turned his focus to the campaign to rejoin. He denied it, saying his strategy was to force a referendum on the Brexit deal, but that sounds like bravado.

This poses the question of how the ABC campaign should fight Brexit over the coming year. Tony Blair, in his article this week, and Peter Mandelson, in his article for The Independent last month, both describe 2018 as the last chance to stop Brexit.

But they both know that is not going to happen unless Jeremy Corbyn bows to the views of Labour Party members, 78 per cent of whom support another referendum, as a YouGov survey for Professor Tim Bale reported this week.

Blair urges Labour to challenge Brexit

On this, we know that Seumas Milne, Corbyn’s chief adviser, thinks the sovereignty of party democracy can go hang: the important thing for him is to protect Corbyn’s anti-establishment message – by respecting the 2016 referendum (and, incidentally, staying on the other side of the question from the ABC elite).

So where does the ABC campaign go next? One route is the Norway option, to press for the UK to stay in the EU single market after Brexit. The problem with that is it would mean accepting free movement of people, and there is no majority in the House of Commons for that. Almost all Conservatives and a large number of Labour MPs want an end to free movement.

However, free movement is going to continue for at least 21 months after Brexit, during the transition period. The precise terms of the transition are now going to be negotiated as the Brexit talks move on to phase two, and this is where the next weak point is to be found in the Brexit defences.

One of the questions to be decided is how long the transition will be. Will it be 21 months to the end of the EU budget period in December 2020, or will it be two years until March 2021? But one critical question on which the ABC campaign should focus is whether that period will be extendable. For the Rebel Alliance, this could be the weak spot in the Brexit Death Star.

So far, both sides have been united in saying the transition period (or implementation phase, as Theresa May calls it) must be “strictly time-limited”. Boris Johnson, for the Brexiters, insists it should be “not a second more” than two years. The technocrats in Brussels don’t want it dragging on either. For them it is untidy, creating a semi-membership status that undermines the unity and consistency of the EU.

But you can see the attraction, for those who want a soft Brexit, of a transition period – in which we stay in the single market and carry on with free movement – that lasts indefinitely. Let us see how it goes, Adonis, Blair and Cable could say, and, if we feel we are not ready for the economic shock of a Canada-style trading status, let us take our time about it.

It might be easier for the ABC campaign to argue for rejoining the EU from the waiting room of transition than from the cold of being right out. Johnson and May will be watching these arguments like hawks, knowing that this could be a way to drop a missile into the heart of Brexit. But it might be possible to muster a majority in the House of Commons for an extendable transition. Labour MPs could say they want an end to free movement but that we must be flexible about it.

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Nor is the Brussels position as firm as it seems. The Irish government is not the only one of the EU27 that would regard a permanent transition as ideal. Many other national leaders could see a flexible period as being in their interests. Cynically, they might even regard Britain in the single market – and paying for the privilege – but excluded from troublemaking in EU Councils, as a perfect outcome. A vassal state, indeed.

The intriguing part of Tony Blair’s anti-Brexit drive this week was his interview with Die Welt and other continental newspapers. He warned the EU27 that the nationalist surge that produced Brexit was a problem in their countries too. He realises the need to lobby EU27 opinion to make life difficult for the Brexiters.

Note, too, that Emmanuel Macron, the French President, this week warned against divisions on the EU side: “Everyone can have an interest in negotiating on their own, and think they can negotiate better than their neighbour. If we do that, it is probable that collectively we will create a situation which is unfavourable to the European Union and thus to each one of us.”

This, then, could be the battle of 2018: not to stop Brexit, but to allow the possibility that a soft exit could be prolonged. For Remainers, this offers the chance of getting stuck in a halfway house, and after a few years people might either decide it is fine or that we should rejoin.