In a cafe in central Derry, just a short drive from the border between Northern Ireland and the Republic, there were expressions of anxiety and indignation, the tone veering from hope to despair to disbelief. How had Northern Ireland come to this, unmoored and uncertain, the voice of its majority ignored, its economy, peace and stability all at risk?

“When the border disappeared there was this massive sense of relief,” said Nicola Herron, 52, a GP and member of Derry Girls Against Borders, which derives its name from the TV comedy Derry Girls, with the blessing of its creator, Lisa McGee. “The thought that it could be reinstated is terrifying.”

That prospect, accompanied by that of economic calamity, edged closer last week when Theresa May failed to win any more concessions from Brussels on her Brexit deal and punted the meaningful vote on it to next month, raising the chance that the UK will crash out of the European Union with no deal at all on 29 March.

It is a prospect that frightens many in Northern Ireland – including the police, who warn of resurgent paramilitary attacks on any hard border infrastructure; and business and farming organisations who warn of crippled trade – but not the Democratic Unionist party (DUP).

Party leaders have said that, in the absence of a renegotiated deal diluting the backstop, they would prefer no deal. Westminster arithmetic may grant their wish, because the government’s majority relies on the DUP’s 10 MPs. But those 10 MPs are now facing mounting calls to shift their position, with an unlikely alliance of nationalists, moderate unionists, grassroots groups, business organisations and the Northern Ireland Office all lined up in opposition to it.

“A nuclear Brexit clock for us is ticking, and ticking loudly,” said Aodhán Connolly, director of the Northern Ireland Retail Consortium. “For us, it really is a countdown to disaster. I can’t say it any clearer. A no-deal for Northern Ireland is a complete disaster.” Scores of companies, plus farming and business organisations, have launched an outspoken campaign in favour of the deal, filling a vacuum left by the collapsed power-sharing executive at Stormont and Sinn Féin’s abstention from Westminster. They say that by staying in the customs union and single market Northern Ireland would thrive – an investment gateway to Europe, the UK and the rest of the world.

Business organisations tried to steer clear of politics during the Troubles but, with the stakes so high, silence was not an option, said Seamus Leheny, policy manager at the Freight Transport Association in Northern Ireland. “When talk of no deal escalated, we felt we had no choice. We didn’t wake up one morning and decide to go head to head with the DUP. But we’re faced with a crisis.”

With Sinn Féin, the SDLP, the Greens, the centrist Alliance party, the Northern Ireland secretary Karen Bradley, grassroots groups, plus the Irish government and the EU rooting for the Brexit deal, the DUP and its leader, Arlene Foster, are under intense pressure. To which it has a two-word rejoinder: no surrender.

The DUP has allies among Conservative party Brexiters, notably Boris Johnson, but otherwise seems friendless, which seems to bother it not a bit.

Facebook Twitter Pinterest Nicola Herron, centre, with other members of the group Derry Girls Against Borders. Photograph: Rory Carroll/The Observer

Sammy Wilson, the party’s Brexit spokesman, has branded business leaders “puppets” of Theresa May, called the taoiseach, Leo Varadkar, a “nutcase” and caused uproar by comparing the deal with the EU to a punishment beating.

DUP imperviousness to criticism has prompted comparisons with the “no one likes us, we don’t care” chant by Millwall FC supporters in England. “The DUP is not softening,” said Jon Tonge, a University of Liverpool politics professor and co-author of a book about the party. “I think they were startled and angered by the positive response to the PM’s Brexit deal from the business and farming sectors. The fracture between the political and business wings of unionism is serious and possibly irreparable.”

Where outsiders see recklessness, the DUP sees logic. Most Northern Ireland voters – 56% – voted Remain in 2016, but most DUP voters voted to leave and they are expected to stay loyal. The DUP is in fact tipped to gain a seat in the next election.

Any economic “dislocation” from a no-deal exit would probably be short-term and manageable, said Peter Weir, DUP member for Strangford in the Northern Ireland assembly. And any economic pain would be in the service of a higher goal – avoiding a backstop that could impose a border down the Irish Sea and inch Northern Ireland out of the UK. May, in other words, must deliver a legally binding new deal, which the EU has ruled out. If that means no deal, so be it.

“We met the DUP recently and don’t believe that there will be a road to a Damascus moment,” said Connolly. “They have their position and believe in it fully. One thing that the past 30 years of politics in Northern Ireland has shown is that nobody can tell the DUP what to do.”