Theresa May can please Brussels or the DUP – not both

The unionists’ threat to vote down the Budget if the Prime Minister breaches their red lines on Brexit underlines the fact that, barring further compromise, the government faces an impossible choice.

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“Not sure how this ends well.” On the eve of Arlene Foster’s trip to Brussels on yesterday, that was the blunt assessment of one senior DUP source of the government’s chances of reaching a solution on the Irish backstop that would meet their red lines.

Two days later, it looks increasingly like things can’t end well for Theresa May’s relationship with the DUP, which is now threatening to vote down the Budget if she agrees to a deal that imposes new regulatory checks in the Irish Sea. It is also briefing that May would probably have to resign should she do so. The future of the confidence and supply deal is at stake.

It is an intervention designed to inflict maximum pain on the Prime Minister. Even before today, the DUP had made ominous noises about the Budget – which falls between the two EU summits at which a deal will be brokered – and Tory Brexiteers are gearing up to make its passage as difficult as possible for the government too. I wrote about both threats at Conservative Party conference last week. The Fixed Term Parliaments Act gives both groups a huge amount of leverage to vote the Budget down without bringing the government down with it should it so wish.

But why the explosion now? The conclusion that the unionists have reached after talks with Michel Barnier is that the Prime Minister is moving towards a deal that will mean checks between on goods travelling between Northern Ireland and Great Britain.

They have made repeatedly clear that this is something they will not accept, and believe in accepting such a compromise that the Prime Minister would be weaselling out of the commitment she made to no new regulatory checks in December. The problem for May is that such a compromise looks inevitable. Downing Street briefed on Monday night that it believed the DUP would accept it, as long as there were only checks at ports in Great Britain. That got much shorter shrift from the unionists than the government were expecting.

If there is to be a deal that the DUP votes for, either it, the European Commission, or May will have to concede on one or more of its red lines. Barnier said this afternoon that new checks on goods moving across Irish Sea were an inevitability. The point of today’s escalation from the unionists is to demonstrate that they are not for turning, no matter what Brussels demands.

Ultimately, the DUP’s only red line – as its MPs wearily repeat – is the integrity of the United Kingdom. No new internal barriers to trade. They mean this in the most basic and literal terms. As far as they are concerned, there is no way May can fudge it. They are tired of her platitudes about "our precious union". But none of this, despite the high drama of these flashpoints, is new information. What’s changed is the fact that Downing Street is now fast approaching the moment when they must decide whether they can get away with pressing ahead with their Brexit strategy while ignoring it – and jettisoning their parliamentary lifebelt.

Patrick Maguire is the New Statesman's political correspondent.