Arlene Foster's DUP is preparing for a "guerrilla war" against any measures that would create what they see as an economic barrier between Northern Ireland and mainland Britain | Charles McQuillan/AFP via Getty Images Unionists plan ‘guerilla war’ against Northern Ireland Brexit plan The Democratic Unionist Party regards the plan to avoid a hard border as ‘vassalage’ for Northern Ireland to the EU.

BELFAST — Theresa May's Brexit could prevent power sharing between Northern Ireland's two communities from ever restarting, senior unionist figures are now saying privately.

While Brexit-watchers are focused on the threat to the Northern Irish peace process from a hardening of the border with the Irish Republic as a consequence of the U.K. leaving the EU, little attention has been paid to possible longer-term consequences of the proposed solution to the border problem: the "backstop."

The Democratic Unionist Party, which votes with May's Conservatives in the House of Commons under a "confidence-and-supply" agreement to keep the Tories in power, is preparing for a "guerrilla war" against any measures that would create what they see as an economic barrier between Northern Ireland and mainland Britain, according to one senior party official speaking on condition of anonymity.

Even if May can get a version of the so-called Irish backstop through the U.K. parliament without DUP votes, the party could use its position as the leading unionist political force in Northern Ireland to try to prevent it from ever being implemented — and in the process block power sharing with nationalist politicians permanently.

“We are going to squeeze their balls until their ears bleed,” the party official said.

"[Theresa May] has said under no circumstances will we hive off Northern Ireland" — Senior aide

Negotiators were taking stock Sunday night after Brexit Secretary Dominic Raab rushed to Brussels for an unscheduled meeting with his opposite number Michel Barnier with the aim of sealing the Brexit divorce deal. At the event, the two failed to conclude an agreement, leaving uncertainty and a tight timetable before a meeting of EU leaders on Wednesday.

The stakes were underlined Saturday in an op-ed by DUP leader Arlene Foster in the Belfast Telegraph following a series of meetings with EU officials and politicians in Brussels last week. She said the future shape of the United Kingdom is now on the line, adding, "This is no game."

“This backstop arrangement would not be temporary. It would be the permanent annexation of Northern Ireland away from the rest of the United Kingdom and forever leave us subject to rules made in a place where we have no say,” she added.

On Sunday, former Brexit Secretary David Davis upped the ante by calling for ministers to rise up and kill off May's Brexit plan. In what amounted to an incitement of Cabinet insurrection, he wrote in the Sunday Times: “It is time for Cabinet members to exert their collective authority. This week the authority of our constitution is on the line.”

No. 10 flatly rejects the accusation that the prime minister will agree to any backstop that creates a border in the Irish Sea. "She is as committed a unionist as anyone," one senior aide said. "She has said under no circumstances will we hive off Northern Ireland."

Brexit impasse

The Irish backstop, which was first agreed to in principle by both London and Brussels in December, is a legal guarantee to prevent the need for north-south border checks whatever the future trade relations between the U.K. and the European Union.

The exact nature of the backstop is still the subject of intense negotiations in Brussels, but under the current proposals, Northern Ireland would need to mirror EU rules for goods and agriculture in perpetuity should no future trade agreement be reached between the EU and the whole of the United Kingdom that also guarantees an open border between Northern Ireland and the Republic.

No. 10 wants the future trading relationship to mean the backstop never comes into force.

Such new rules would need to be brought into U.K. law as and when they are introduced in Brussels. This can happen in Westminster — by either the U.K. government or parliament — or in Northern Ireland itself, by the devolved administration, namely the power-sharing executive or the Stormont assembly. Those bodies have been suspended since January 2017 following a breakdown in trust between unionist and nationalist parties.

Under the central pillar of the Good Friday Agreement, the main nationalist and unionist parties share power, each effectively wielding a veto over the other.

Given the outright opposition to the current backstop plan by the main unionist party, the DUP, the prospect of new EU regulations being accepted by Stormont if they are different to the rules on the mainland is close to zero — even if the democratic institutions are revived in the future.

“If the unionist community feels they have been screwed on Brexit, it makes a return to devolution harder — much harder" — Senior DUP official

To ensure the U.K. sticks to a backstop that includes regulatory divergence between Northern Ireland and the rest of the U.K., there are only two options — either direct rule from Westminster to enforce different rules for Northern Ireland, or full alignment with EU rules for the whole of the U.K. “Vassalage for Northern Ireland or vassalage for the U.K.,” in the words of the senior DUP official speaking on condition of anonymity.

“The thing people don’t realize is Brexit, devolution and the peace process are interlinked,” the official said. “If the unionist community feels they have been screwed on Brexit, it makes a return to devolution harder — much harder.”

Violence unlikely

Dublin believes the time has come for May to face down the DUP to protect the open border with the Republic.

The Irish government argues Brexit is an international treaty between London and the European Union that cannot be held to ransom by one party from one part of the U.K., either now or in the future. They also point out that the DUP does not represent a majority in Northern Ireland. Indeed, it is a source of some concern among unionists that for the first time in Northern Ireland's history there is no longer a unionist majority in the Stormont assembly.

The prospect of loyalist violence in response to a backstop being forced on Northern Ireland is also downplayed by both nationalists and unionists. Infrastructure at the border is an easy target — regulation is harder to attack.

Two high-profile figures, one nationalist, the other unionist, predicted “kicking and screaming” from the unionist community but no return to violence.

Inside the DUP, however, they are digging in for the long haul.

“When Theresa May was running in wheat, [DUP leader] Arlene was seeing her dad crawl back home after being shot,” the party official said. “Let’s be blunt, we’ve been through worse than this. Do they really think we’ll just give up?”

The DUP plans to fight the U.K. government tooth and nail in Westminster before bringing the fight back home to Northern Ireland if they have to.

They believe the deal they agreed with the Tories after the prime minister lost her majority in the 2017 election, coupled with the Fixed Term Parliament Act in Westminster — which makes it harder for opposition parties to trigger elections — gives them real power by allowing them to vote against the Tories without the risk they topple the government in the process.