Why a Brexit deal might fail even if May agrees to Corbyn’s custom union demands

The trouble for May is that if Corbyn is able to say he has moved her on Brext, the size of the Conservative rebellion will almost certainly significantly increase.

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Is Theresa May preparing to make Jeremy Corbyn an offer he can't refuse, caving to his demand for a customs union? Eurosceptic Conservatives certainly fear she might be, and their worries have made it to the front page of the Telegraph.

The real problem is the government's four policy objectives for Brexit, which are that the whole of the United Kingdom remains a part of the same shared customs and regulatory area after Brexit, maintenance of the status quo on the Irish border, frictionless trade and protection of rules of origin status for British-made goods, and an independent trade policy. Of these four, three can only be met within a customs union and with considerable regulatory alignment, while a meaningful independent trade policy can only be achieved via a customs and regulatory border in the Irish Sea.

The reason for Brexiteers to reject May's withdrawal agreement – which would take the United Kingdom out of the single market, the common agricultural policy, the domestic reach of the ECJ, and the commons fisheries policy; a hard Brexit by any reasonable definition – is precisely because it is a customs union in name only, the final conclusion of May's Thesaurus Brexit, where she tries to win over her own party's ultra by finding different names for the same thing.

But of course, for Corbyn to be able to declare victory, he needs to be able to rip away the pretence and to say that he has moved May on Brexit. The trouble is that in doing so, he almost certainly significantly increases the size of the Conservative rebellion, because Conservative MPs in marginal seats without religion about the withdrawal agreement are nervous about the fact that their voters dislike both May's deal and Corbyn. For those MPs, adding the two together feels like a one-way ticket to a large third-party vote for Brexit, and a Labour gain through the middle. So any offer to Corbyn loses Conservative votes in parliament.

The question is: can Corbyn deliver enough Labour votes to make up for that? At the moment, it's possible. But we haven't really had a policy debate about what a customs union means, about the challenges of Brexit it leaves unresolved and about the fact that a customs union Brexit is still a rock-hard Brexit. (It's just also a hard Brexit that foregoes the major policy lever that makes Brexit worthwhile from a Conservative perspective.)

There might well be a narrow majority in this parliament for a customs union Brexit today. But after a few weeks of conversation about what that form of Brexit would actually mean and the actual trade-offs contained in it, I'm not convinced there would be any more of a majority to take the United Kingdom out of the EU this way, than there is a majority for May's customs union in name only.

Stephen Bush is political editor of the New Statesman. His daily briefing, Morning Call, provides a quick and essential guide to domestic and global politics.