BELFAST, Northern Ireland — Michelle O’Neill was forced to greet visitors this week in a drab upstairs meeting room at the rear of the Stormont Parliament Buildings in Belfast, its faded posters and scattered chairs a stark contrast to the classical grandeur of the chambers at the front of the complex.
A leader of the Irish nationalist Sinn Fein party, Ms. O’Neill had just vacated her office as deputy first minister of Northern Ireland’s government after the first minister, Paul Givan, a member of the main unionist party — that is, the main party supporting Northern Ireland’s current status as part of the United Kingdom — abruptly resigned. Under the power-sharing agreement that governs the territory, she automatically lost her post as well.
But if the upheaval turned Ms. O’Neill into a temporary vagabond, it also served to underline a momentous political shift in Northern Ireland: Assuming that current polls hold, Sinn Fein, with its vestigial ties to the paramilitary Irish Republican Army and fervent commitment to Irish unification, will become the largest party in the Northern Ireland Assembly after elections scheduled for May.
That could catapult the 45-year-old Ms. O’Neill into the post of first minister, and it helps explain why Mr. Givan quit when he did.
His Democratic Unionist Party is desperate to rally its voters before the election. Its most emotive issue is the North’s trade status in the wake of Brexit, which is governed by a complex legal arrangement known as the Northern Ireland Protocol. Unionists complain that the protocol, which requires border checks on goods passing between Northern Ireland and mainland Britain, has driven a wedge between the North and the rest of the United Kingdom.
By pulling their leader out of Stormont, the Democratic Unionists are trying to put pressure on the British government, which is in the process of renegotiating the protocol with European Union. Unless the trade rules are radically overhauled, unionists say, they will not return to the government and Northern Ireland’s on-again off-again experiment in power-sharing will collapse.
“We’ve had enough of being promised that this issue would be dealt with,” said Gordon Lyons, 35, a Democratic Unionist who serves as economy minister in the government and who will stay in his position until the election. “There’s a general sense that we unionists are always being asked to suck it up.”
Ms. O’Neill dismissed Mr. Givan’s exit as a “reckless stunt.” It came days after another unionist minister, Edwin Poots, declared that the government would stop inspecting agricultural goods coming in from Britain, a violation of the protocol. A judge ruled that the checks must continue until the issue was decided in court.
“They’ve been on the wrong side of the Brexit debate,” Ms. O’Neill said. “Now they’re bringing their dysfunction into this building.”
Behind the theatrics, however, is a deadly serious contest for the future of Northern Ireland, one that could reverberate widely, destabilizing not just the island but also Britain’s relations with the European Union and the United States.
Nearly a quarter century after the Good Friday Agreement ended the sectarian violence known as the Troubles, Brexit has scrambled Northern Ireland’s politics. Few want a return to the bloody 30-year guerrilla war that set mostly Catholic nationalists and republicans, seeking unification with Ireland, against predominantly Protestant loyalists and unionists, who want to stay in the United Kingdom.
But the fallout from Brexit has left unionists angry and divided, and it has tilted the political landscape in favor of Sinn Fein, which opposed Brexit and seeks ever closer ties between the north and south of Ireland.
“This does feel like a critical juncture,” said Katy Hayward, a professor of politics at Queen’s University in Belfast. “We can’t avoid the fact that 100 years after its creation, Northern Ireland has fundamentally changed.”
If Sinn Fein does win the largest number of seats — it is currently eight points ahead of the Democratic Unionists in polls — the most likely scenario would be a prolonged negotiation as the two parties tried to figure out how to live with each other. But some experts said they doubted the Democratic Unionists could ever take part in a government with a Sinn Fein representative as first minister.
As a practical matter, the first minister and deputy first minister have equal powers in overseeing the government — an arrangement designed to force parties from opposing traditions to work together. But in the identity politics of Northern Ireland, symbolic details matter.
Unionists complain that Sinn Fein vetoed their plans to plant a rose bush at Stormont last year to mark the centenary of the establishment of Northern Ireland. Nationalists point out that the unionists opposed legislation that would give the Irish language similar status to that of English, as Welsh has in Wales.
“It’s about a sense of loss,” said Monica McWilliams, an academic and former politician who was involved in the 1998 peace negotiations. “The unionists say, ‘If this is going to be good for the Irish economy, it’s going to be bad for us up north.’”
On its face, the Northern Ireland Protocol would not seem to have the visceral power of issues like language. It is a technical arrangement that grew out of a deal between London and Brussels to avoid resurrecting a hard border between Ireland, an E.U. member state, and Northern Ireland, which left the European Union as part of the United Kingdom. To achieve this, it requires checks on goods flowing across the Irish Sea from mainland Britain to the North.
Mr. Givan’s party enthusiastically supported Brexit, and when Prime Minister Boris Johnson struck the deal on the protocol, they grudgingly went along with it. But as the checks have begun to be enforced, unionists say they have imposed an onerous burden, with one widely quoted analysis estimating that Brexit adds 850 million pounds, or $1.15 billion, a year in costs. Other experts cast doubt on those figures and point out that Northern Ireland has bounced back more quickly from the pandemic than much of Britain.
Still, there is a palpable sense of betrayal at the hands of Mr. Johnson. First, he promised the unionists that the protocol would not disrupt trade across the Irish Sea. Then he told them that Britain would drive a hard bargain with the European Union, scrapping the protocol, if necessary, to remove barriers.
Now, however, Mr. Johnson, embattled by his own scandals at home, is wary of igniting a trade war with the European Union. He also recognizes that stirring up tensions over Northern Ireland would antagonize President Biden, who takes a particular interest in the preservation of the Good Friday Agreement.
When Mr. Johnson’s hard-line trade negotiator, David Frost, resigned last December — in part over concerns about this softer stance on the protocol — he was replaced by a more emollient figure, Liz Truss, the foreign secretary. While the negotiations remain tough, Britain and the European Union are stressing progress and seem less likely to come to blows.
“The least difficult option for Boris Johnson is to sacrifice Northern Ireland,” said David Campbell, chairman of the Loyalist Communities Council, which represents a group of pro-union paramilitary groups that vehemently oppose the protocol.
Some of those groups were suspected of instigating clashes with the police in April last year when tensions over the protocol first boiled over. Mr. Campbell insisted in an interview that was not the case, though he warned that if London were to cut another deal with Brussels, “The message it would send is that the only thing that works is violence.”
In the short term, the protocol’s biggest threat is to the Democratic Unionists, who are being challenged by rival parties on both their right and left. “How do you reward these people for all their blunders?” Mr. Campbell said.
The Democratic Unionist Party, which was founded by the Rev. Ian Paisley during the height of the Troubles, has cycled through leaders and lurched sharply to the right as it struggles to shore up its base.
“Someone in Europe needs to wake up to the reality that they are not doing this to assist the peace process,” Mr. Poots declared in the Assembly on Monday. “The political element of the peace process has had a bomb put in it, and it hasn’t been by terrorists, it has been by the European Union.”
Such fiery words pose a problem for both the European Union and Britain. While London could resort to imposing direct rule on the North — as it has during previous breakdowns in relations between the Northern Irish parties — that would further inflame tensions. To make the protocol work smoothly, both sides need a functioning administration in Belfast to set up and enforce much of the border checks.
“It can, in theory, be overridden, but we shouldn’t underestimate the political costs of doing this,” said Raoul Ruparel, a former special adviser to the British prime minister on Europe. “The U.K. government tramping into Northern Ireland just doesn’t seem to be a reasonable request.”