Only in Northern Ireland could you get a party which wins in elections on the back of a pledge not to take its seats if elected.
It doesn't matter if there is a hung Parliament and its votes are vital, Sinn Fein will be no shows in the next Parliament.
Abstentionism, as the policy is known, leaves Sinn punching seriously below its weight. While it may not impact on the party's vote on May 6, foregoing the influence which its five seats could exercise in a closely balanced Westminster risks damaging the party's longer term credibility.
Politics is all about power and leverage - and Sinn Fein is voluntarily giving up one of its main weapons.
As Peter Robinson put it at his manifesto launch, a hung Parliament is an opportunity that comes to small parties once in a political lifetime. If it comes now, as the bookies and many polls predict, Sinn Fein will miss out on it.
There was some sort of skewed logic to this policy while an active and armed IRA backed Sinn Fein's political demands with its considerable paramilitary muscle.
Then Sinn Fein could claim to be part of a revolutionary movement negotiating British withdrawal at the point of a gun and hoping to achieve it soon.
Then the IRA was regarded, in Martin McGuinness's words, as "the cutting edge" of the republican struggle. Those days are long gone. Sinn Fein not only takes seats in Stormont, Leinster House and councils, but is committed to building a stable and prosperous Northern Ireland.
Yet abstentionism lingers on like a bad smell to remind us of long-dead policies smouldering away in some untended corner of the republican psyche.
The absurdity of the situation becomes clear if we consider what would happen if this policy was imposed on us by London.
What if, of all the regions of the UK, Northern Ireland was legally excluded from Westminster?
Most taxes would be set in London without local input and foreign and security policy would be handled the same way. Our local representatives would be rewarded with allowances to travel to London and lobby; they would get perks and flats and would be allowed occasional audiences with the Prime Minister, but no votes.
Such a system would be rightly described as colonial. "No taxation without representation" was the slogan which sparked the Boston Tea Party and the American Revolution.
In the case of Sinn Fein, exclusion from the tax-making authority is not imposed by imperialists, it is a voluntary choice which Gerry Adams parades as the principle of "active abstentionism".
Adams now says that he could not join other MPs in taking an oath of allegiance to the Queen.
Yet when the idea of replacing the Parliamentary oath with a Stormont-style undertaking to faithfully discharge the office of MP was mooted, it was rejected by republicans.
The real theoretical underpinning of abstentionism rests on what Dr Martin Mansergh, long Fianna Fail's point-man with Sinn Fein, a piece of "preposterous nonsense" which holds that the IRA Army Council is the "real" government of Ireland. The IRA constitution once held that all other assemblies are illegitimate and their "main tasks are treasonable".
It first became a real issue after Sinn Fein's landslide victory in the all-Ireland election of 1918. Then the party won 73 out of the 105 seats which Ireland was then entitled to send to Westminster.
Sinn Fein withdrew and formed a parliament in Dublin, the first Dail. This laid the foundation for Irish independence as well as the unintended consequence of civil war and partition that followed.
The virtues of that policy can be debated, but it is undeniable that, even with Sinn Fein then in such a strong position, it had a downside in denying nationalism influence at Westminster.
Winston Churchill recorded in his diary that he "rejoiced in the blessed abstentionism of Sinn Fein" as he scanned the Irish benches, which had been dominated by nationalists for so long.
Churchill feared that Irish MPs could have co-operated with radicals to drive policy towards Irish independence and impede the war effort. Instead, the 26 unionists acted as the voice of Ireland in the Commons, laying the foundations for partition.
In republican ideology, the 1918 election was regarded, for most of the 20th century, as the last legitimate, all-Ireland election. After partition, the remnants of the first Dail handed on their mandate to the IRA Army Council which supposedly held onto it until 1986.
Then General Tom Maguire, the last survivor of the first Dail, took umbrage at the decision of Sinn Fein to take seats in the Dail and passed on the mandate to Republican Sinn Fein and the Continuity IRA.
It is more like the Da Vinci Code than democratic politics. It lost all semblance of internal logic in May 1998 when the whole of Ireland voted simultaneously for the Good Friday Agreement, recognising partition and British jurisdiction unless and until it was overturned in a referendum.
Abstentionism is an anachronism which reduces the value of Sinn Fein MPs and disenfranchises the constituencies they represent. How long will it be before the voters demand a change?