We’re not even at Christmas and already battle lines have been drawn in the clash over who will lead the mainly Catholic Social Democratic and Labour Party into the next general election.

Current leader, Assembly member and Foyle MP Mark Durkan, steps down in February and the SDLP faces a tough choice between two big hitters to replace him.

Alasdair McDonnell, the party’s deputy leader, represents old-style, traditional and moderate nationalism – the kind which saw former SDLP leader and joint Nobel Peace Prize winner John Hume dominate the Irish nationalist agenda for almost a generation.

To date, McDonnell’s greatest triumph was when he snatched the supposedly ultra-safe Unionist Westminster seat of South Belfast at the 2005 general election, which was previously held by Orange clerics Martin Smyth and Robert Bradford. The latter was murdered by the IRA in 1981. McDonnell was able to capitalise on the growing fragmentation within unionism.

However, it is by no means certain that he will replace Durkan. He faces a formidable opponent in Stormont executive minister Margaret Ritchie, who has already demonstrated her leadership qualities by putting the brakes on funding which was destined for hardline loyalist areas. Ritchie enjoys the support of the SDLP’s youth wing and has predicted a united Ireland within her lifetime. She is viewed as the standard-bearer of middle-class Catholic and respectable Irish republicanism.

While both would be credible leaders, the deciding question is likely to be: who is best placed to rescue the party from the electoral oblivion that Sinn Fein could inflict on it.

Sinn Fein has virtually eclipsed the SDLP by its core vote – middle and upper-class, well-educated Catholics. At the 2005 Westminster election and the Northern Ireland Assembly election two years later, these traditionally core SDLP supporters deserted the party in droves for the darker green republicanism of Sinn Fein.

Tactically, whoever the new SDLP leader is, he or she may only be able to outmanoeuvre Sinn Fein by throwing their party wholeheartedly behind a merger with the Republic of Ireland’s largest nationalist party, Fianna Fail, currently led by beleaguered Prime Minister Brian Cowen.

A merged, moderate republican party, operating both north and south of the border, might be able to market itself as an all-island alternative to Sinn Fein. The new united organisation could position itself as a centre left movement and make a point of contrasting its policies with Sinn Fein’s hard left agenda.

Sinn Fein’s main problem is its lack of engagement with its power-sharing partner at Stormont, the Democratic Unionist Party. To halt the slide in support to the more hardline Traditional Unionist Voice movement, the DUP shifted further to the right. This has placed an increasing strain on Northern Ireland’s ruling coalition.

The Ulster Unionists, gaining some ground after aligning with David Cameron’s Conservative, will want an SDLP leader with whom they can do business.

Although the next Stormont election is not scheduled until 2011, it is looking increasingly likely that the DUP-Sinn Fein government will not be able to last until then. If the executive does collapse, there could be a snap Assembly election in February next year.

Given the severe splits among the Unionists, Sinn Fein could even emerge as the largest party in the Assembly – an achievement which would secure it the coveted post of First Minister.

A British general election is due in little more than six months and Sinn Fein might go into the campaign as the lead nationalist party before the new SDLP leader has had chance to settle in. Currently, Sinn Fein has five MPs to the SDLP’s three. However, the latter still refuses to take its seats in the House of Commons, making it a virtually insignificant Westminster force.

If David Cameron moves into Number 10 Downing Street, there are likely to be places in a Conservative government for UUP MPs and peers. If the new SDLP leader were come to some sort of arrangement with the Tories and the UUP, an SDLP MP might join them – perhaps as a junior Northern Ireland minister.

Meanwhile, Sinn Fein has suffered some setbacks in the Irish Republic, such as the recent referendum endorsement of the Lisbon Treaty, for instance.

In the murky world of Irish political mergers and deals done behind closed doors, perhaps an amalgamation of Sinn Fein and the SDLP should not be entirely ruled out. This would see a return to the sort of situation in the 1960s, when one grouping spoke for the vast majority of republicans. This was the late and not unlamented Irish Nationalist Party.

Those who snort with derision at the suggestion of such a merger might pause and cast their minds back to 1981. Then, with the republican hunger strikes at their height, who would have taken the possibility of Sinn Fein participating in a partitionist parliament at Stormont seriously? Would have thought that Ian Paisley would one day be Northern Ireland First Minister – and in partnership with republicans? In the traditionally volatile arena of Irish politics, change is always in the wind.