ON Thursday a crucial election takes place for the Northern Ireland Assembly – an election whose outcome could tear another hole in the tottering foundations of our so-called United Kingdom. Commentators in the north are billing this the most important election since the Good Friday Agreement, or even the most crucial since the Ulster “province” was created after Irish independence in 1921.

For on Thursday it is possible that Sinn Fein will win a plurality of the seats in the assembly, overtaking the Democratic Unionist Party (DUP). Even more earth-shattering, it is possible the combined (if fractious) Unionist parties might, for the first time ever, find themselves in a minority in a Northern Ireland parliament or assembly. Add that to a hard Brexit (with its unwelcome reinstatement of border controls), and you have all the makings of a political crisis. A crisis that could accelerate the exit of Northern Ireland from what’s left of the UK.

First a necessary caveat: elections to the Northern Ireland Assembly are conducted under the Single Transferable Vote (STV) system that we use here in Scotland for local elections. Predicting the final share of seats under STV is always difficult – the more so in Northern Ireland where the precise tactical transfer of votes between parties will determine the final outcome. Predictions are also further complicated by the fact that this week’s election is the first held with reduced seats (from 108 to 90) which will squeeze the smaller parties.

In last May’s assembly election, eight parties won seats. On the Unionist side: the DUP, the Ulster Unionist Party, and the hard-line Traditional Unionist Voice – a split from the DUP that refuses to be in joint government with Sinn Fein. On the united Ireland wing were Sinn Fein and its long-standing rival, the Social Democratic and Labour Party (SDLP), which shares with the SNP a vision of a civic nationalism. Straddling the constitutional issue were the Alliance Party (think LibDem), the Greens, and the far-left People Before Profit.

Why another election so soon after last year’s? The post-1969 Troubles arose from the unwillingness of the old Unionist oligarchy to accept reform and the dismantling of the Protestant Ascendancy inside the Northern Ireland Bantustan. But violence as a political strategy on both sides merely entrenched divisions and reduced the province to chaos. A generation of stalemate went by until political exhaustion produced a rickety compromise in the Good Friday Agreement signed in 1998.

This ushered in de-militarisation and a power-sharing assembly. Tacitly, political and constitutional divisions were frozen. The assumption was, on all sides – including on the UK mainland – that this truce would hold until demography finally led to a majority Catholic population in the north peacefully federating with the Irish Republic. But suddenly and unexpectedly this compromise has started to unravel.

Not, I must point out, that a return to violence is likely. The generation who took to the gun has all but passed. However, politics in the north has to move on soon because the stasis inherent in the power-sharing system has run its course. Northern Ireland desperately needs a proper, contested, democratic politics – especially now that the lunacy of a hard Brexit threatens to reintroduce the border into Irish affairs just when it is least needed.

Under the Good Friday devolved system, Unionists and Republicans have to agree to get anything done – fair enough, given what went before. But since this system was put in place, we have seen the 2008 banking crash and a Tory government at Westminster imposing a lunatic austerity programme. As a result, much more needs doing in Northern Ireland – economically and in terms of social reform – than is possible under the political stasis imposed by power-sharing.

This stasis has been reinforced by the gradual concentration of representation in the hands of the right-wing DUP and leftish Sinn Fein. And where there is political stalemate there is often corruption. The north has been hit by a series of seedy political scandals in recent years involving everything from housing maintenance bribes and alleged child abuse to insider dealing in the sale of bank assets. Matters finally boiled over when the DUP’s leader and the assembly first minister, Arlene Foster, bungled a green-energy initiative that could end up costing the UK and Northern Ireland taxpayers upwards of a billion quid.

At the start of this year, as the full extent of the “cash for ash” fiasco became apparent, Martin McGuinness (then Sinn Fein’s deputy first minister) demanded Foster’s resignation or his party would quit the administration. There was a degree of opportunism here as Sinn Fein had been under pressure from its base for seeming too cosy with the DUP. Equally, Brexit has played a role in Sinn Fein’s calculations. While Northern Ireland voted heavily for Remain, the DUP was cheerleader for Leave.

When Foster refused to stand down as First Minister, Sinn Fein retaliated by failing to appoint a replacement for McGuinness, thus forcing a fresh election under the Good Friday rules. Tactically, Sinn Fein hopes to become the largest party after Thursday. This will give it the right to nominate the first minister – a psychological blow to Unionism in the north. The latest poll has the DUP and Sinn Fein neck and neck in support, though current odds favour the DUP when it comes to seats. If the DUP squeezes back in as the largest party, with Foster still at the helm, expect Sinn Fein to refuse to re-join the administration. Under Good Friday rules, that will necessitate yet another election. If nothing changes then, we are looking at direct rule from London.

Whatever Thursday’s immediate outcome, the constitutional question is back in play in Northern Ireland. In both Scotland and Northern Ireland, Brexit is the real catalyst. Irish PM Enda Kenny has just called for easy Northern Ireland entry into the EU to be written into any UK Brexit agreement, should the local electorate ultimately decide on Irish unity. If Dublin wants this the UK Government will have to sign up to that. Also, expect Sinn Fein’s new leader in the north, Michelle O’Neill, to oppose the re-introduction of border controls.

Meanwhile, the sharp-eyed among you will have noticed that there are no Labour Party candidates standing in the Northern Ireland Assembly elections. Indeed, Labour has just expelled two of its activists for trying to run on Thursday. Strange considering that Labour’s London Mayor, Sadiq Khan, came all the way to Perth to lecture us that we are “better” for being a United Kingdom and that “there’s no difference between those who try to divide us on the basis of whether we’re English or Scottish and those who try to divide us on the base of our background, race or religion”.

Labour is happy to accuse the SNP of dividing the UK yet has always tacitly supported Irish unity. In fact, the official Labour Party Irish Society is currently tweeting support for SDLP candidates in Thursday’s election. Which means, I presume, that both Sadiq Khan and Kezia Dugdale approve of the SDLP’s “aspiration to share political union with the rest of the [Irish] island”, as stated in its election manifesto. Or is Labour just being hypocritical towards Scotland, as usual?