LONDON — Northern Ireland was carved out of the Irish Republic a century ago to protect the rights of its predominantly Protestant, pro-British population. But on Friday, the largest Irish nationalist party, Sinn Fein, was on the cusp of being declared the territory’s largest party, a political watershed in a land long torn by sectarian violence.
With much of the vote in legislative elections counted on Friday evening, Sinn Fein was on track to win the most seats in the Northern Ireland Assembly, a distinction that will allow it to name the first minister in the government.
The significance of the election lies less in political privileges than hard-fought history: A nationalist party at the helm in Northern Ireland will kindle new hopes for Irish unity, but it could also sow a return to unrest between Catholics and Protestants in a territory where delicate power-sharing arrangements have kept the peace for more than two decades.
It is a remarkable coming-of-age for a party that many still associate with paramilitary violence.
“For nationalists who have lived in Northern Ireland for decades, to see Sinn Fein as the largest party is an emotional moment,” said Diarmaid Ferriter, professor of modern Irish history at University College Dublin. “The very idea of leading a government in Northern Ireland would once have been repugnant to it.”
Across the United Kingdom, local election results on Friday were handing some setbacks to Prime Minister Boris Johnson, in what was widely seen as a test of the damage to him and his Conservative Party from a swirling scandal over lockdown-breaking parties at Downing Street.
But it was in Northern Ireland where the results were carrying the most sweeping potential for change.
Sinn Fein’s victory has deeply unsettled the unionists, who have declined to say they will take part in a government with a Sinn Fein first minister. That could lead to a breakdown of Northern Ireland’s parliament, known as Stormont, and paralysis in the government. Some even fear a flare-up of the violence between Catholics and Protestants that the peace accord ended after the 30-year guerrilla war known as the Troubles.
Sinn Fein made its electoral gains with a campaign that emphasized kitchen-table issues like the rising cost of living and health care, and that played down its totemic commitment to uniting the North and South of Ireland — a vestige of its ties to the Irish Republican Army.
The shift will push the Democratic Unionist Party, which favors Northern Ireland’s present status as a part of the United Kingdom, into second place for the first time since the 1998 Good Friday Agreement, which created the system under which unionists and nationalists share power.
Among the other likely big winners in the election was the Alliance, a centrist party that aligns with neither the nationalists nor the unionists. Analysts said the party’s candidates had drawn votes away from “soft unionists,” suggesting that the sectarian conflicts of the past are less resonant, particularly with younger voters, than everyday concerns like housing, jobs and health care.
“A plurality of voters in Northern Ireland say they are not nationalist or unionist,” said Katy Hayward, a professor of politics at Queen’s University in Belfast. “Now there seems to be momentum behind that view.”
“The overriding point Sinn Fein is making is, ‘We want to be in government,’” Professor Hayward said. “That is welcomed by people who are fed up by the dysfunction of the government.”
In so-called first-preference votes, which were reported on Friday evening, Sinn Fein won 250,388 votes, the Democratic Unionist Party won 184,002, and the Alliance won 116,681. Under the territory’s complicated voting system, candidates with the largest number of votes automatically win seats in the assembly.
But voters can express additional preferences, and seats are allocated according to the parties’ share of votes. That means that the final number of seats won by Sinn Fein and other parties will not be clear until Saturday.
For all the symbolism, the victory was as much about disarray in the unionist movement as the rise of the nationalists. Unionists have been divided and demoralized since Brexit, largely because the Democratic Unionist Party signed off on the British government’s negotiation of a hybrid trade status for Northern Ireland, known as the protocol.
The arrangement, which imposes border checks on goods flowing from mainland Britain to Northern Ireland, has triggered a backlash among unionist voters, many of whom complain that it has driven a wedge between them and the rest of the United Kingdom. The British government, eager to mollify the unionists, is weighing legislation that would throw out parts of the trade protocol. But it has yet to act.
Such a move would ratchet up tensions with the European Union and possibly even spill into a trade war. It would also antagonize the United States, which has warned Britain not to take steps that could jeopardize the Good Friday Agreement — a pact negotiated under the auspices of the Clinton administration.
President Biden, who frequently talks about his Irish roots and staunchly opposed Brexit, has raised Northern Ireland’s status in meetings with Mr. Johnson. He has also asked his staff to reiterate his concerns about the issue to British officials.
While unionists point to the trade protocol as the source of their problems, analysts said that Brexit, which a majority of voters in Northern Ireland opposed, was at the root of the divisions within the movement.
“It’s Brexit that’s casting a shadow over Northern Ireland,” said Bobby McDonagh, a former Irish ambassador to Britain. “It’s not the protocol, which is actually an attempt to solve the problems caused by Brexit.”
An aggressive new push for Irish unity could also threaten the peace. Sinn Fein officials play down the prospect of that, noting that it is up to the British government to decide whether to schedule a referendum asking people in Northern Ireland if they want to remain in the United Kingdom or unite with the Republic of Ireland.
A majority of people in the South would also have to vote in favor of unity, something that experts say is also likely to take years. Sinn Fein has increased its support in the Irish Republic as well, with a similar appeal to voters on bread-and-butter issues like housing prices. It is now Ireland’s main opposition party and stands a chance of being in the government after elections scheduled for 2025.
“Sinn Fein is now in the unique position — that it is an all-Ireland party,” Professor Ferriter said. “But if it is to be successful, given that its fundamental objective remains Irish unity, it has to give momentum to that effort.”
For all its evolution into a mainstream party, analysts say Sinn Fein still bears traces of its militant roots. It remains highly centralized, with little of the internal debate or dissent that characterize other parties.
In the United States, where many in the Irish diaspora embrace the nationalist cause, the party’s supporters took out ads before St. Patrick’s Day in The New York Times and other newspapers that promised “Irish unity in our time.”