Sinn Fein looks set to become the first Irish nationalist party to win the most seats in Northern Ireland, as UK PM Boris Johnson faces a pivotal mid-term popularity test.
Polls have opened across the UK in local and regional elections that could prove historic in Northern Ireland and heap further pressure on embattled Prime Minister Boris Johnson.
The contest for the devolved assembly in Belfast on Thursday could see a pro-Irish nationalist party win for the first time in the troubled history of the British province.
The results, which are expected from Friday, could have huge constitutional implications for the four-nation UK's future, with predicted victors Sinn Fein committed to a vote in the province on reunification with Ireland.
Polls open at 0600 GMT for councils in Scotland, Wales and much of England, with Johnson facing a potentially pivotal mid-term popularity test.
Poor results could reignite simmering discontent within his ruling Conservatives about his leadership, after a string of recent scandals.
Johnson's position has been put in jeopardy because of anger at revelations of lockdown-breaking parties at his Downing Street office and a cost-of-living crisis.
Heavy losses could revive calls among Tory MPs to trigger an internal contest to oust Johnson as party leader and from power.
'A sea change'
The polls should point to whether the main opposition Labour party poses a serious threat, as it tries to make inroads across England despite defending the many gains it made at the last local elections in 2018.
Labour is bidding to leapfrog the Conservatives into second place in Scotland, behind the pro-independence Scottish National Party (SNP), and remain the largest party in Wales, where 16 and 17-year-olds are eligible to vote for the first time.
The contest for Northern Ireland's power-sharing assembly is set to capture attention, after numerous polls put Sinn Fein ahead.
A University of Liverpool poll reported Tuesday it remained on target to win comfortably with over a quarter of the vote. The pro-UK Democratic Unionist Party (DUP) and cross-community Alliance Party were tied for second.
Deirdre Heenan, professor of social policy at Ulster University, said there was a feeling the election "really is momentous". "It will be a sea change if a nationalist becomes first minister," she said.
Sinn Fein - the IRA's former political wing - has dialed down its calls for Irish unity during campaigning, saying it is "not fixated" on a date for a sovereignty poll, instead focusing on the rising cost of living and other local issues.
Party vice president Michelle O'Neill has insisted voters are "looking towards the future" with pragmatism rather than the dogmatism that has long been the hallmark of Northern Irish politics.