Sinn Fein leader Mary Lou McDonald, left, and Deputy leader Michelle O Neill arrive at the election count centre in Belfast , Northern Ireland Counting is continuing across Northern Ireland in the Assembly elections. AP
A Sinn Fein win would be historic because Unionist parties have led the legislature since Northern Ireland was formed as a Protestant-majority state in 1921.
An election victory would bring the party's ultimate goal of a united Ireland a step closer. It's a milestone for a party long linked to the Irish Republican Army, a paramilitary group that used bombs and bullets to try to take Northern Ireland out of U.K. rule during decades of violence involving Irish republican militants, Protestant Loyalist paramilitaries and the U.K. army and police.
But Sinn Fein has kept unification out of the spotlight this year during a campaign that has been dominated by the skyrocketing cost of living. Party leaders have indicated there will be no immediate moves for a referendum on a united Ireland, and no constitutional change would take place until voters decide on it.
With more than two-thirds of 90 seats counted so far, results on Saturday showed that Sinn Fein has 23 seats, while the Democratic Unionist Party, which has been the largest party in the Northern Ireland Assembly for two decades, has 21.
The centrist Alliance Party, which doesn't identify as either nationalist or unionist, has seen support surge and is set to be the other big winner of this year's local U.K. elections. It has 15 seats so far.
While a Sinn Fein win would be a historic shift that shows diminishing support for unionist parties, it's far from clear what happens next.
Under a mandatory power-sharing system created by the 1998 peace agreement that ended decades of Catholic-Protestant conflict, the jobs of first minister and deputy first minister are split between the biggest unionist party and the largest nationalist one. Both posts must be filled for a government to function, but the Democratic Unionist Party has suggested it might not serve under a Sinn Fein first minister.
The DUP has also said it will refuse to join a new government unless there are major changes to post-Brexit border arrangements, known as the Northern Ireland Protocol.
The post-Brexit rules, which took effect after Britain left the European Union, have imposed customs and border checks on some goods entering Northern Ireland from the rest of the U.K. The arrangement was designed to maintain an open border between Northern Ireland and EU member Ireland, a key pillar of the peace process.
But it angered many unionists, who maintain that the new checks have created a barrier between Northern Ireland and the rest of the U.K. that undermines their British identity.
In February, the DUP's Paul Givan resigned as first minister as post-Brexit tensions triggered a fresh political crisis in Northern Ireland.
Polling expert John Curtice, a professor of political science at the University of Strathclyde, said the Northern Ireland results from the local elections Thursday are a legacy of Brexit.
``The unionist vote has fragmented because of the divisions within the community over whether or not the Northern Ireland Protocol is something that can be amended satisfactorily or whether it needs to be scrapped,'' he wrote on the BBC website.
Persuading the DUP to join a new government and trying to press the EU to agree to major changes in post-Brexit arrangements will pose a headache for British Prime Minister Boris Johnson, he added. Johnson's own Conservative party lost at least 450 seats in this week's local election.
Sinn Fein vice-president Michelle O'Neill said it was imperative for political leaders to come together to form an Executive _ the devolved government of Northern Ireland _ next week.
There is ``space in this state for everyone, all of us together,`` she said Saturday. ``There is an urgency to restore an Executive and start putting money back in people's pockets, to start to fix the health service. The people can't wait.''
O'Neill has said with regards to Irish unification, there would be no constitutional change until voters decide on it. Sinn Fein leader Mary Lou McDonald indicated Friday that planning for any unity referendum could come within the next five years.
The full results of Thursday's local election, which uses a system of proportional representation, were expected later in the weekend.
Northern Ireland's new legislators will meet next week to try to form an executive body. If none can be formed within six months, the administration will collapse, triggering a new election and more uncertainty.