In agreeing a Brexit deal without the support of the DUP, Boris Johnson has taken a bold risk that leaves him with the narrowest path towards passing it at Westminster. Even with the DUP's 10 MPs, his whips estimated that the deal could only pass narrowly if almost every Conservative, most of the 21 expelled Tories and a number of Labour MPs backed it.
Without the DUP, Johnson will need to keep the number of Conservative rebels to a minimum and to win over up to two dozen Labour MPs. Few at Westminster this week put the number of potential Labour backers above 12, so Johnson will need a further inducement to make up the numbers.
Under the Benn Act, Johnson is obliged to request a three-month delay to Brexit if he fails to pass the deal at Westminster. Downing Street confirmed on Thursday that Johnson has asked the EU to rule out granting any extension to Britain's membership but the 27 leaders declined to do so.
The prime minister would not comment on whipping arrangements for Saturday's vote but he has warned Conservative Brexiteers in the past that they would lose the whip and be barred from standing as candidates at the next general election if they failed to support any deal he brought back from Brussels.
Both Dublin and London believed after a phone call between Johnson and Leo Varadkar on Wednesday morning that the proposals on consent would be enough to satisfy the DUP. But after hours of meetings in Downing Street and at Westminster on Wednesday, the DUP remained convinced that they provided inadequate safeguards for unionists.
The DUP’s decision to withhold support leaves them isolated from Conservatives at Westminster and they are already facing criticism from the Tory press. One senior DUP figure said this week that it would be easier for them if Johnson could pass the deal without their help.
Their electoral calculation may be that by standing firm against the deal, they will be less vulnerable to the Ulster Unionist Party (UUP) and Jim Allister's Traditional Unionist Voice (TUV). But if they are responsible for the deal's failure to win parliamentary approval and for a no-deal Brexit, they will be also be responsible for the consequences that follow.