The Democratic Unionist party (DUP) is facing two options, one bad, the other worse. The trick will be deciding which is which. It can reject the Brexit deal that British and EU negotiators are discussing in Brussels, on the assumption it proposes a border down the Irish Sea.

Rejection would rupture its relations with Boris Johnson and could sink the deal in the Commons, shunt the UK towards no deal, wreck Northern Ireland’s economy and add support for a united Ireland.

Or Arlene Foster and her 10 Westminster MPs could endorse the deal and smooth its passage through parliament, then face howls of betrayal from rival unionists who will try to engrave Brexit on the DUP’s tombstone.

Foster kept the party’s options open in a holding statement last Friday, which reiterated opposition to a backstop or any measure that would “trap” Northern Ireland in the EU. It also urged a “balanced and sensible” deal in Northern Ireland’s “long-term” economic and constitutional interests – ambiguous language that left wiggle room. “We will judge any outcome reached by the prime minister against the criteria above,” it concluded.

Less ambiguous language came on Saturday from the party’s Nigel Dodds, who told the Italian newspaper La Repubblica: “No, it cannot work, because Northern Ireland has to remain fully part of the UK customs union.”

The party’s impossible choice revolves around Downing Street’s suspected drift back to a Northern Ireland backstop, leaving the region in a customs partnership with the EU. This would be a potential economic boon for Northern Ireland, with trade flowing in every direction, yet politically toxic for the DUP because of its “blood-red line” of no border in the Irish Sea, deeming that a violation of Northern Ireland’s position in the UK.

The party backed leave, neither expecting it to win nor anticipating the profound implications for Northern Ireland

Rival unionists are already crying foul. “I fear that Northern Ireland is being offered up by Boris Johnson as the sacrificial lamb to save Brexit for the rest of the UK,” said Jim Nicholson of the Ulster Unionist party. It would be a “fundamental assault upon our position within the United Kingdom”, thundered Jim Allister of Traditional Unionist Voice.

The DUP has made big U-turns before, such as sharing power with Sinn Féin from 2007, and electorally thrived. Conceivably it could do so again. But its identity and success rest on branding other unionists as pusillanimous wimps who cannot be trusted with the union. To have that turned against them could be costly at the polls. The alternative is also grim: kick the PM where it hurts and risk holding the bag for a no-deal exit that alienates Northern Ireland’s business sector and pushes centrist voters towards Sinn Féin.

The dilemma results from some grievous miscalculations. For Eurosceptic kudos, the party backed leave in the 2016 referendum, neither expecting the Leave campaign to win nor anticipating the profound implications for Northern Ireland.

They compounded the error by depicting checks in the Irish Sea as an existential threat, rather than a technical issue with manageable political ramifications. Wielding leverage over Theresa May’s government blinded the DUP to danger and engendered a giddy sense of unprecedented power. “Unionism has never had so much influence in parliament,” Foster boasted in 2017.

That seems a long time ago.