BELFAST, Northern Ireland — John Finucane was 8 years old when he witnessed masked men smash down his front door with a sledgehammer and assassinate his father as the family sat at the dinner table.

It was a typically dreary Sunday evening in Belfast on Feb. 12, 1989, when the attackers sprayed bullets into Patrick Finucane, a prominent lawyer, as his wife and three children watched in horror.

The killing was one of the most high profile of "the troubles" — a conflict that plagued Northern Ireland for three decades until a 1998 peace deal brought hope of reconciliation.

Today, Finucane — 39, the same age as his father was when he was slain — is at the center of an ugly electoral fight that has revived bitter rhetoric, threats of widespread violence and talk that the United Kingdom itself might break apart.

"We were having Sunday dinner, like so many other families do, when two gunmen came in and shot my father 14 times," said Finucane, who is the mayor of Belfast and running for lawmaker in Parliament in London. "I'm sure people can imagine just how traumatic that day was."

"It was very difficult growing up without a father," added Finucane, a tall, authoritative presence who slips into a folksy, everyman rapport when chatting with locals on the campaign trail.

John Finucane out in the Ardoyne area of north Belfast. Charles McQuillan / for NBC News

Now he worries that certain aspects of this election are "a throwback to those dangerous and divisive times."

In England, Scotland and Wales, the nationwide election this Thursday will decide whether Conservative Prime Minister Boris Johnson can consolidate power and drive through his hard-line Brexit deal.

In Northern Ireland, a unique corner of the U.K., the vote has revived altogether more ancient and existential demons.

Deirdre Heenan, a professor at Ulster University, called it "one of the most toxic, divisive elections" the region has seen in recent years.

"It would be difficult to overestimate its importance," she said. "The stakes are so high in terms of peace, in terms of our stability and in terms of our economic future."

'The blood of our innocents'

It comes at a time when most people here are desperate to move on from "the troubles."

The conflict was mainly between Roman Catholic "republicans," who identify as Irish and want to unite with the Irish Republic south of the border, and Protestant "loyalists," who feel British and want to remain in the U.K.

From 1969 to 1998, more than 3,600 people — mostly civilians — were killed. Violence flared between the Irish Republican Army, an outlawed terrorist organization fighting the British state, and pro-British paramilitary groups such as the Ulster Defence Association.

The bloodshed ended with the Good Friday Agreement of April 10, 1998, which former U.S. Sen. George J. Mitchell, D-Maine, helped broker.

It wasn't a perfect peace.

Parts of Belfast and elsewhere are still festooned with the paraphernalia of division: flags, murals, segregated schools, and 20-foot fences with gates that lock at night — the last designed to keep certain Protestant and Catholic enclaves from attacking each other.

Some nights, Molotov cocktails and bricks are still launched into surrounding streets.

All the same, the Good Friday Agreement allowed two communities with seemingly irreconcilable claims over this territory the size of Connecticut to live in something approaching peace.

Finucane's campaign exemplifies how Brexit has disturbed the fragile balance of this accord.

Last month, anonymous activists put up gruesome banners in Belfast alleging the Finucane family is "steeped in the blood of our innocents." The banners claimed that Patrick Finucane was a member of the IRA, the deadliest group in the troubles.

Free Derry corner in the Catholic Bogside area of Derry. Charles McQuillan / for NBC News

Finucane did represent members of the IRA, but also their loyalist opponents. While three of his brothers were alleged members, his family and police say he never was himself.

His cases often criticized the actions of the U.K. government in Northern Ireland. And an inquiry later found the British state was guilty of "shocking levels of collusion" in his murder by loyalist paramilitaries, as then-Prime Minister David Cameron put it in 2012.

His youngest son, John, was also pictured on the banners. The mayor is running for lawmaker with Sinn Fein, a socialist party that was once the political wing of the IRA and strives for a united Ireland.

The party has denied it, but the police and intelligence services say Sinn Fein is still linked to the IRA. Like many paramilitary organizations in Northern Ireland, the IRA has largely turned away from violence and criminality and toward politics, according to the authorities.

Police say that this group and others still have access to some weapons, and some of its members are involved in crime, but after agreeing to the 1998 peace deal the IRA and others are not planning to carry out terror attacks.

During his campaign for Sinn Fein, Finucane spent an afternoon with NBC News canvassing at a 1980s shopping mall in the predominantly Catholic working class neighborhood of Ardoyne.

Finucane on the campaign trail alongside Michelle O'Neill, Sinn Fein's deputy leader. Charles McQuillan / for NBC News

"The banners were very sinister. I think they were a very deliberate and coordinated attempt to try to intimidate me," said Finucane, who has stated he opposes all forms of violence. "But I'm focusing on a very positive message because this election is too important to be fought from the gutter."

That "positive message" is all about Brexit.

Like in Scotland, most people in Northern Ireland voted against leaving the European Union. The 2016 referendum was ultimately carried by pro-Brexit votes in England and Wales

Finucane vehemently opposes it, eager to discuss Brexit's potential to damage the economy and stability, rather than talk about past violence or sectarianism.

Many people here also want to move on. A poll published this summer found 50 percent identify as neither British unionist nor Irish nationalist. And the centrist Alliance Party enjoyed a surge in local and European elections earlier this year.

People here will tell you they want to tackle cross-community problems, such as the ailing health care and education systems in what is one of the poorest parts of the U.K.

"Nobody wants to revert back," said Natasha Frame, 29, a nursing assistant with two young children. "I sure as hell don't want my kids experiencing what I did growing up."

However, Finucane's opponents say he is far from blameless when it comes to reanimating specters of the past. Among his doorstep canvassing team is a former IRA bomber whose botched attack killed nine Protestant civilians in 1993.

"To have this person so highly visible in the John Finucane campaign I think is very hurtful to the victims and I think it is a rather callous thing to have done," said John Kyle, a local politician with the Progressive Unionist Party.

John Kyle, stood in Belfast's Cluan Place, says the election has brought out ugly language from all sides. Charles McQuillan / for NBC News

Kyle's is a smaller political party with its own historic links to the Ulster Volunteer Force, a pro-British loyalist paramilitary group responsible for hundreds of murders during the troubles.

Though containing criminal elements, UVF leaders are attempting to steer the group toward peaceful community work, police say.

Kyle says his party was "historically linked" and today provides "political analysis" to the UVF, which he describes as a "post-conflict group." He rejects violence and does not have a paramilitary background, he says.

Kyle lamented that both communities "have taken the kid gloves off" during this election, "with some very brash nationalistic messages behind the rhetoric."

The convicted bomber on Finucane's canvassing team was among more than 400 people released from prison early under the terms of the Good Friday Agreement, which said these people could be freed if their groups laid down their guns.

When asked about his canvassing team, Finucane said, "I think that all political parties … need to be cognizant as to how we use people who were ex-prisoners."

"But ex-prisoners have a role to play in politics," he added. "I don't want to live in a society where we deny people the opportunity to fulfill their ambitions through a democratic and political output."

'Not everything has to be sectarian'

Even if Finucane wins his seat, he will not sit in Parliament in London. All Sinn Fein lawmakers boycott the legislature because they see Britain as an illegitimate presence in their homeland.

None of the U.K.'s main political parties — Conservative, Labour and Liberal Democrats — have a significant presence in Northern Ireland's 18 electoral constituencies, so the political landscape is dominated by local groups.

Sinn Fein's boycott means the right-wing Democratic Unionist Party has been Northern Ireland's dominant voice for the past two years. Its 10 lawmakers have been given an unprecedented amount of power, invited into an informal coalition by Conservative ex-Prime Minister Theresa May.

The DUP, which has sought to downplay its own paramilitary associations, is anti-abortion and anti-same sex marriage and features politicians with creationist views.

Cluan Place, a Protestant enclave in Belfast. Charles McQuillan / for NBC News

In 1986, some of its present-day lawmakers helped establish an organization that smuggled weapons into Northern Ireland. These rifles and ammunition were passed onto paramilitary groups that used them to kill dozens of people, including many civilians.

Party leader Arlene Foster was criticized in 2017 for meeting with the leader of the Ulster Defence Association, another banned terrorist group, and other loyalist organizations backed the party's candidates in that year's nationwide election.

With the DUP emboldened and Sinn Fein absent in London, this lopsided landscape is compounded by Northern Ireland's regional assembly being suspended for almost three years over various disagreements.

And now many fear a surprise electoral pact will polarize politics further.

Northern Irish voters usually choose among least five political parties. This election, however, some of these parties are standing aside for each other, leaving just one pro-Irish and one pro-British candidate to contest several battleground seats head-to-head.

The politicians say this is all about stopping the other side's Brexit plans, although many politicians and academics fear it will merely lead to each community lining up behind the candidate for its respective tribe, deepening sectarian divisions.

Colum Eastwood, leader of the Social Democratic and Labour Party, a left-wing nationalist group, dismissed accusations that his and other politicians' actions would further polarize an already divided society. He told NBC News any cooperation on his part is about trying to stop Brexit.

"It's a sensible approach and it's actually a progressive approach," he said. "Not everything has to be sectarian. I sometimes wonder why people who call you sectarian are so obsessed with that?"

Graham Warke stands in the Fountain estate. Charles McQuillan / for NBC News

Nevertheless campaigners on the ground say the temperature has risen.

Sixty miles from Belfast is Northern Ireland's second city, which unionists call Londonderry and nationalists call Derry. Graham Warke, a councilor with the Democratic Unionist Party, said while he was putting up posters, a carload of men shouted "tiocfaidh ár lá" at him — an Irish phrase meaning "our day will come" often used by republican paramilitaries.

A former British soldier, Warke now works at a youth center in an enclave called the Fountain, home to some 300 Protestants and walled by 20-foot fences.

He says Northern Ireland has "moved on so, so much" from the days of violence. Back then his mother, Jeanette Warke, 72, who runs the youth center, used to smuggle cigarettes, candy and butter over from the then-more abundant Republic. "If the youth club wasn't here, many more of the people in this area would have ended up in jail," she said.

Still, shouting "tiocfaidh ár lá" is not taken lightly here and can result in a criminal conviction. It unnerved Warke enough for him to report it to the police.

'The betrayal act'

Brexit has created a dilemma for pro-British unionists. Many supported leaving the E.U. but they despise Johnson's exit plan, which they brand "the betrayal act."

"I don't remember in my adult life there being so much hatred and so much polarization among the unionist community," activist Jamie Bryson, 29, said sitting in a dimly lit backroom of the East Belfast Constitutional Club.

Loyalist Jamie Bryson opposes the Northern Ireland peace deal signed in 1998. Charles McQuillan / for NBC News

This dispute centers on borders and identity. In 1998, the peace deal was built on deliberate ambiguity: People could identify as British or Irish, and the border with the Irish Republic was all but erased.

But a hard-line Brexit means there will likely have to be border infrastructure somewhere. Putting it between Northern Ireland and the Irish Republic would be seen by many as a threat to their Irishness, and could be attacked by smaller dissident republican groups that opposed the 1998 peace deal, police warn.

Polls suggest that this, coupled with Brexit's predicted economic pain, would be enough to persuade a majority in Northern Ireland to vote for a united Ireland — breaking up the U.K.

Perhaps fearing this outcome, Johnson wants to put the customs border between Northern Ireland and the rest of the U.K. Unionists hate this, too, worried it would push them away from London and toward Dublin.

"I don't think any group would need to orchestrate civil disobedience because the organic explosion of grassroots loyalist anger would be enough," Bryson said. "Unfortunately those things can get out of hand and spill over. Once that genie gets of the bottle it's difficult to stop."

Bryson describes himself as a "political extremist," and is widely seen as outside of mainstream politics. But in recent months he has attended a handful of closed-door grassroots meetings where similar drastic sentiments were aired.

"Northern Ireland is a divided society where the very identity of the state is contested," he said. "It is us versus them and I want my side to win. That's as basic and as brutal as I can put it."

The feeling of disloyalty is so strong because Johnson is meant to be on their side. His Conservative and Unionist Party has historically been a protector of this uneasy kingdom of nations.

"Don't use bombs, don't use bullets, but go for a protest," said Alex McClements, 50, who works in security and lives yards from the frontier in the Fountain enclave. "I'd be the first one to confront the police, that wouldn't bother me. And if a policeman has to get injured, so what? We have to fight for what we want."

A gate out of the Fountain estate that is closed every night to keep communities separated. Charles McQuillan / for NBC News

Recent polls show most people in Britain would be willing to see Northern Ireland leave the U.K. if that's the price for achieving Brexit, the ultimate manifestation of English nationalism.

"That's why Boris' deal is called the betrayal act — you can only be betrayed by your friends," said Robert McCartney, 55, an industrial services manager and unionist activist.

Equally unpalatable for unionists would be a victory for Labour leader Jeremy Corbyn. In the past he has advocated a united Ireland, and invited IRA members to Parliament in 1984, three weeks after the group attempted to assassinate then-Prime Minister Margaret Thatcher with a bomb.

'We need to move on'

Northern Ireland has a population of less than 2 million, and it's common to find people with a direct connection to the decades of violence.

Patricia and John Burns say they are still searching for justice after their father, Thomas, was killed by the British Army in 1972 in what was the most violent year of the troubles.

While claiming impartiality, the British army and local police killed far more Catholics than Protestants during the troubles, most notably fatally shooting 13 unarmed civilians on Bloody Sunday in 1972.

Killed the same year, Thomas Burns was unarmed and not linked to paramilitaries, according to the family. They say he was in fact a former member of the British Royal Navy who was trying to leave a pub with his friends.

"The hardest thing for me to swallow is that my daddy was an innocent man," Patricia Burns, 53, said as she sat in the front room of her terraced house on a gray, stormy day in west Belfast. "The army treated anybody who lived in a nationalist area as a legitimate target."

Patricia Burns says her family has never recovered from her father's death Charles McQuillan / for NBC News

She says it wasn't just the loss of her father that left the family with irreparable scars, it was the allegations by the press and the police in the immediate aftermath that he was a militant, a claim that was never proved and later dropped.

"We were brought up under this cloud where we were the criminals," she said. "That made us feel that he must have been doing something, there's a kind of guilt attached to that."

Her brother is in no doubt Brexit and this election are dredging up dark elements of the past.

"It's the snide and the nasty remarks," John Burns, 52, a civil servant, said. "They are still digging up history: You said this 30 years ago, you said this 50 years ago. We need to move on from that."

In another part of west Belfast, Margaret Caldwell and Pauline Scott are also searching for answers. Their brother, Gerard Gibson, was killed that same year at age 16.

"When the Good Friday Agreement was brokered, there was nothing could be done for our Gerald. But I just thought it would be a better life for the grandchildren," Caldwell, 62, a school lunch aide, said.

"It hasn't quite turned out like that," she said. "We don't have peace, we have a peace. And it could kick off at any moment."