For the DUP it is the best of times and the worst of times. Theresa May’s reliance upon the votes of the party’s 10 MPs at Westminster has seen more money for Northern Ireland and a critical role in the shaping of influence of Brexit. But the nearer the DUP gets to the centre of the British power, the greater the damage to Northern Ireland’s place within the UK.

Irish nationalist and unionist influence at Westminster was unwelcome in the 20th century; that trend looks set to continue in the 21st. The Conservative party, already resentful of the Barnett formula of fiscal redistribution to Belfast, is quietly furious at the “ransom” of an additional £1bn being allocated to Northern Ireland as part of the “confidence and supply” agreement with the DUP.

DUP influence over Brexit is its greatest weakness. Put simply, it has no good outcomes. There are four possible Brexit scenarios for the DUP, all of which are deeply damaging to the party and, by extension, unionism in Northern Ireland.

The first is that May’s government will look to moderates in the Labour party and elsewhere to push through an agreement with the EU that includes a Northern Ireland backstop – opening the way for possible regulatory divergence between Northern Ireland and the rest of the UK in order to avoid a hard border on the island of Ireland.

The confidence and supply agreement between the government and the DUP would collapse, but May may gamble that the DUP would still vote with the government most of the time. In the medium term, a new election would seem inevitable. But May could reckon on doing reasonably well at the polls if her government survived long enough to deliver a plan for Brexit that avoided significant damage to the UK economy.

The second scenario is that DUP objections to an Irish backstop contributes to a no-deal Brexit. The Treasury and the Bank of England agree that while such a scenario would be very damaging for the UK economy as a whole, it would be especially disastrous for Northern Ireland. The DUP’s part in creating the inevitable recession that would follow would not enamour English voters of making future sacrifices to keep Northern Ireland in the union. It would also seriously damage its reputation at home.

The third prospective outcome is that a UK government – with support from cross-party centrists – decides to renege on previous commitments to leave the customs union and the single market for an indefinite period. This is the best outcome for Northern Ireland’s economy but it still would have negative implications for the DUP, which has long campaigned for the UK to leave both. Nigel Dodds, the leader of the DUP in the House of Commons, has repeatedly promised that the UK would be able to strike its own trade deals and be richer for it.

The final option is a People’s Vote with an opportunity for voters to opt for the UK to remain in the EU. The DUP would again make the argument that Brexit is best but would once more lose the vote in Northern Ireland, very likely by a larger margin. Leakage of unionist votes to the remain cause might have long-term political effects. Voters who didn’t listen to the arguments of Mike Nesbitt when he was leader of the Ulster Unionist party, or to the Alliance party – both of whom advocated a remain vote – might now permanently lose faith in the DUP.

The DUP has never produced a detailed policy paper on Brexit. Instead it deploys the hopeful but empty nationalistic lexicon of Boris Johnson. Some of its representatives, including party leader Arlene Foster, are aware of Northern Ireland’s vulnerability to any interruption or new barriers to EU trade. This has not prevented the party from developing close links with the right of the Conservative party. In its “blood red” commitment to remaining an integrated part of the UK, it has mistaken English nationalism for British patriotism. Most English leave voters say they would sacrifice the union in order to win Brexit. Against its better interests, the DUP has embraced those who view Northern Ireland as a disposable appendage, if not a liability, to England’s future prosperity.

The DUP has been quick to attack any unionist who has strayed from the mantra that the UK must leave the customs union. Last month DUP MP Jeffrey Donaldson accused Dr John Kyle, the Progressive Unionist party Belfast city councillor, of disloyalty for suggesting that the UK government should drop its current Brexit plans in favour of a customs union between the EU and the UK. It is difficult to imagine the concerns of Scottish Conservative leader Ruth Davidson about Brexit being dismissed in such simplistic terms. In the months after the Brexit referendum, Nesbitt observed that union was most secure when “the border was less obvious and less important”; a hard border means increased popular support among Catholics for hard Irish nationalism. Unionists will increasingly rely upon support from the growing number of Catholic voters for Northern Ireland to remain in the UK rather than opting for Irish unification.

The outcome of the Brexit process is unknown. But it has already done significant harm to unionism in Northern Ireland. One thing is for certain: things can only get worse for the DUP.

• Edward Burke is director of the Centre for Conflict, Security and Terrorism at the University of Nottingham