Why does Sinn Fein fight for Westminster seats its MPs won't occupy?

Sinn Fein voters would feel betrayed if their MPs took their seats at Westminster.

They elect them on the basis that they have promised not to do so.

The policy dates to 1917, when the party's first elected members adopted the abstentionist position.

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Sinn Fein's first MP - Joseph McGuinness - would not have been able to take his seat, even if he wanted to.

He was elected from a prison, while serving time for his role in the 1916 Easter Rising against British rule.

McGuiness had campaigned on a pledge not to take his seat in the British parliament.

Sinn Fein's Constance Markievicz made history as the first woman to be elected an MP.

She is often airbrushed from British memory, not least because she chose not to occupy her seat in the Commons.

What is the objective of abstention?

The clue is in the party's Irish name: Sinn Fein, loosely translated as "We Ourselves".

In its 1918 manifesto, the party pledged to create an Irish republic "by withdrawing Irish representation from the British parliament".

It went on to defeat the Irish Parliamentary Party and established the first Dail Eireann (Ireland's lower house) in 1919.

Today, the party, which holds seven of Northern Ireland's 18 Westminster constituencies, continues to decline its seats to demonstrate that it does not recognise British rule in Northern Ireland.

The parliamentary oath is another issue - as Irish republicans, Sinn Fein MPs will not swear allegiance to the British monarch.

A century on, many question the rationale of abstention, when participation in the legislative assembly in Northern Ireland has brought dividends.

Sinn Fein's Michelle O'Neill made history this year when she became the first nationalist to be elected First Minister of Northern Ireland.

The party is also topping the polls in the Republic, with its leader hoping to become the next Taoiseach (prime minister).

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It is the largest party at Stormont and in local government here and will be looking for a hat-trick.

Becoming the largest Northern Ireland party at Westminster would be yet another significant political moment.

More Northern Ireland seats would be empty than occupied in the UK parliament.