Dr David Shiels is a contemporary historian and College Research Associate at Wolfson College, Cambridge.

This Thursday, a by-election will be held in the constituency of West Tyrone. The poll was triggered by the resignation of Sinn Fein’s Barry McElduff, whose short tenure as MP ended in controversy following a video he posted on social media on the anniversary of one of the worst incidents of the Troubles. West Tyrone has been regarded as a safe Sinn Fein seat since it was first won by the party at the 2001 general election. In 2017, McElduff topped the poll with just over 50 per cent of the vote. Following his resignation, there had been some talk of fielding a unity or ‘victims candidate’ against Sinn Fein, but these plans came to nothing, and five parties are contesting the election.

A rural constituency, bordering Donegal in the Republic, West Tyrone includes the towns of Omagh and Strabane. The affairs of the constituency are often overlooked in Northern Ireland, and the by-election will attract little interest from the media in London. This is unfortunate, because the issues being debated in West Tyrone are central to the debate over Brexit and the future of the Union. Because of Sinn Fein’s policy of abstention from the Westminster Parliament, the constituency has not had a representative who has taken their seat in the Commons for 17 years. Despite Sinn Fein’s claim to be ‘standing against Brexit & Tory austerity’, their absence from Parliament makes it easier for the Conservatives (with the help of the DUP) to command a majority in the Commons.

Recently, Sinn Fein’s policy of abstention has been the subject of sharper criticism from nationalist voices in the Republic. Micheál Martin, the leader of Ireland’s main opposition party, Fianna Fáil, has said that it was ‘totally illogical for Sinn Fein to say they can stay out of Westminster given that Brexit is the single greatest issue facing our generation’. The Taoiseach, Leo Varadkar, has urged Sinn Fein to consider entering the Commons to ‘make things better for Ireland’ over Brexit.

Sinn Fein’s leaders have firmly rejected any departure from their policy, and for good reason. The suggestion that the party might come to the rescue of Jeremy Corbyn is an intriguing one, but the plan would probably not work out as intended (and in any case the numbers are not there to defeat the Government in a confidence motion). For one thing, the party understands the way that English politics works: were Sinn Fein seen to be helping to defeat or bring down the British Government, the backlash against them would probably propel the Conservatives back into office with a majority.

Moreover, Sinn Fein’s policy of abstention is not just about objecting to the oath or affirmation of allegiance to the monarch. For Sinn Fein, it is about a long-term strategy of making Westminster seem irrelevant in Irish affairs. Irish Republicans look back to the 1918 General Election – when Sinn Fein won 73 out of 105 Irish seats – as the democratic basis for a sovereign, independent Ireland. In the tradition of their predecessors, who refused to recognise Westminster and set up an alternative Parliament, the revolutionary Dáil Éireann, in 1919, Sinn Fein’s MPs are campaigning for the right to speak in the present-day Irish Parliament. Whatever the short-term attractions of going to London, the party knows that as soon as they concede the principle that their votes in the House of Commons matter, it becomes harder to defend the logic of abstentionism at all.

The restoration of direct rule would further advance Sinn Fein’s cause. While direct rule was often derided as a ‘colonial’ form of government when it operated in the period before 2007, the democratic deficit would become more apparent in the context of English Votes for English Laws. How is it right that British MPs can vote on matters affecting only Northern Ireland, especially when there are parts of Northern Ireland that do not have any representation in the Commons? The ‘West Tyrone Question’ is a bit like the West Lothian Question in reverse, and there are really no satisfactory answers to it.

From the Unionist point of view, Sinn Fein’s abstentionism presents a serious challenge to their arguments about the legitimacy and authority of the British Parliament. At the 2017 General Election, seven out of 18 constituencies effectively opted out of the British Parliament, at a time when that Parliament is making decisions that will affect them in the long term. But it is easy to forget this as the DUP celebrate their current influence at Westminster. Sinn Fein knows that the DUP’s influence will not last forever, and they also know that opposition to the confidence and supply deal in Britain works to their advantage.

At the same time, the prospect of Direct Rule – or even a more limited form of Direct Rule by stealth – will raise uncomfortable questions for Unionists, too. How would they react to Westminster passing legislation on same-sex marriage, or reform of the laws on abortion? Will they resort to the familiar anti-Westminster rhetoric of the past, helping to make Sinn Fein’s arguments for them? All of these questions are a reminder that Westminster is relevant to the future of Northern Ireland, notwithstanding Sinn Fein’s claim to the contrary. Until last year, Sinn Fein had at least been prepared to make the Northern Ireland Executive work. But they know that they can continue their policy of ‘double abstention’, staying out of the institutions in Belfast and London, without damaging their long-term strategy. Unionists need the institutions to work.

Ultimately, Northern Ireland’s constitutional status will not change unless the people vote for Irish unity in a border poll. That said, by continuing to win support for their policy of abstention, Sinn Fein are already winning a moral victory over the Unionists. As far as West Tyrone is concerned, the argument for the Union may be lost already. The policy of abstention represents a greater challenge to the Union than Brexit.