BBC Northern Ireland
The 2010 Westminster election will be Northern Ireland's 20th full election since 1987.
The fight for the nationalist/republican vote in Northern Ireland, unlike the fight for unionism, has been contained to just two parties: the Social Democratic Labour Party (SDLP) and Sinn Fein.
The SDLP is a party tracing its roots back to the Northern Ireland Civil Rights Movement of the late 1960s; Sinn Fein, formed in 1905 is an older party that has changed dramatically in size, structure and ideology.
The SDLP have always been committed to exclusively peaceful means for achieving their goal of a united Ireland; by contrast, Sinn Fein was linked with the work of the Provisional IRA since the outbreak of the Troubles.
Since the Good Friday Agreement however, the party has sought to distance itself from the aggressive IRA campaign, and have since surpassed the SDLP at the polls on a regular basis.
In the 1987 Westminster election, the first held in Northern Ireland since the Anglo-Irish Agreement, the SDLP confirmed their dominance in the nationalist/republican vote, receiving 22% of the overall share.
Sinn Fein surpassed the DUP in terms of overall share, with the third largest share of overall votes, with just over 107,000.
John Hume, leader of the SDLP, used his party mandate of working peacefully towards a united Ireland, and began secret talks with Sinn Fein leader Gerry Adams over a possible end to IRA violence.
Their progress, and subsequent talks with the British government, led the way to a peace forum in Northern Ireland in 1996, for all-party talks.
The SDLP took 21 seats at the forum; Sinn Fein were close behind on 17.
In the 1997 Westminster election, Sinn Fein president Gerry Adams won back his West Belfast seat from the SDLP, and Martin McGuinness dislodged William McCrea of the DUP in Mid-Ulster.
Attention turned to negotiations over a possible way forward for Northern Ireland.
Prolonged talks eventually led to the signing of the Good Friday Agreement in Belfast in 1998, with the promise of fresh elections to fill the Stormont Assembly through a form of proportional representation.
John Hume and Gerry Adams argued to their electorate that the deal marked a "staging post" to Irish unity, and the first step in wresting power away from London.
Partly due to this positive message of progress from their leaders, Sinn Fein received a substantial share of the votes in the 1998 election: 17% overall, equating to 18 seats.
2001 - 2005
At the Westminster election of 2001, Sinn Fein overtook the SDLP on the nationalist side for the first time.
Michelle Gildernew capitalised on a split unionist vote in Fermanagh South Tyrone to win the seat, and Pat Doherty won West Tyrone from the UUP.
Sinn Fein had doubled their Westminster seats to four.
As the stability of the Stormont arrangements and the long-term objectives of the UUP and SDLP were called into question, voters on both sides now turned to the Sinn Fein and DUP alternatives.
In 2003, Sinn Fein won almost a quarter of the overall vote and 24 seats in the Assembly.
They gained another seat in 2005 at Westminster, this time taking Newry and Armagh from the SDLP.
Later that year, the IRA announced it would begin decommissioning, a task completed by September. The way seemed clear for Sinn Fein and the DUP to speak for the first time.
In November 2006, the DUP and Sinn Fein announced that, following ongoing talks with the British and Irish governments, they had reached an agreement for power-sharing to be restored at Stormont.
The St Andrew's Agreement that emerged served to amend some aspects of the 1998 Agreement, and committed Sinn Fein to full support of the PSNI and the devolution of policing and justice powers to Northern no later than two years from the date of agreement.
Fresh elections were called for March 2007; once again Sinn Fein put severe pressure on the SDLP, and strengthened their lead over the moderate nationalists to 12 seats.
They captured 26% of the overall vote, well ahead of both the SDLP and UUP.
In 2009, Sinn Fein confirmed their sharp rise in fortunes in Northern Ireland by topping the European election poll, helped by a split vote in unionism with the appearance of Jim Allister's Traditional Unionist Voice (TUV) on the ballot.
Sinn Fein's relatively rapid rise to prominence as a political party has posed serious challenges to their nationalist rivals.
The 2010 election has forced the SDLP to explain the remaining ideological differences between the two parties.
The SDLP are emphasising the fact that they, unlike Sinn Fein, take their seats at Westminster, and so are present for any crucial votes concerning Northern Ireland.
Sinn Fein justify their abstentionist stance by pointing to the mandate they have been given by the voters, and claim that progress to a united Ireland will involve consolidating as much power in Belfast as possible, rendering Westminster largely insignificant.
The SDLP have rejected overtures from Sinn Fein for an electoral pact, but it remains to be seen how big a threat they can pose to their rivals on 6 May.