IT may go without saying that whoever is appointed leader of the Scottish Labour party on Saturday will become immediately the most important Labour figure in Scotland.

What may be less obvious is the important role the successful candidate will play in the future of the Labour party throughout the United Kingdom and in the outcome of next May's General Election.

How Scotland votes on May 7 could determine whether the Queen invites a Tory or Labour prime minister to move their goods and chattels into No 10. Every vote that is not for the Labour Party throughout the UK makes David Cameron's return more likely. It is no exaggeration to contend the role of Scottish Labour leader has never been more important.

The polls suggest Jim Murphy, East Renfrewshire's MP, will win but nothing can be taken for granted under Labour's Byzantium rules until the final vote is counted. Though MPs and MSPs have voted overwhelmingly for Mr Murphy and, according to telephone canvassers, party members have backed him by at least three votes to one, the trades union bloc vote is unknown and hard to predict.

The smaller trades union, like Usdaw and Community, have backed Mr Murphy. The mighty Unite has not. One wag, and it may be apocryphal, observed Unite's members would not have known three candidates were standing in the leadership election until they saw the ballot paper, such has been Unite's promotion of Neil Findlay, their favoured son.

Mr Murphy, who would be Labour's best hope in the forthcoming election battle, knows only too well how the trades unions might can effect the outcome of the leadership election. Although David Milliband, who was beaten by his brother, Ed, comfortably won the support of elected members and party members, he didn't win well enough in these sections to see off the union votes.

The immediate challenge for the new leader is building an attractive, credible, optimistic team able to make the political weather. The polls at the moment make pretty grim reading for Labour. To see off an SNP rout at the General Election they have no time to waste if the trend is to be reversed.

How the losers and their allies react to the outcome of the ballot will be very important. The last thing the Labour party needs is a bout of internal bickering, taking their eye of the big picture and further disenchanting the electorate. Nothing is more unappealing than a political party publicly settling scores. It should not be too much to hope the Scottish Labour party and the wider Labour movement has at least learned that lesson.

Johann Lamont fired a pretty damning salvo across the Labour party's bows when she stepped down from the leadership to return to the backbenches at Holyrood. Many colleagues were furious and offended but while it may not have been her intention she has created space and may have liberated her successor. In future Scottish Labour will be dancing to its own tune.

Kezia Dugdale is the hot favourite to win the ballot for deputy leader of the Scottish party. Self-appointed anti-establishment members of the party, including a handful of MPs, have supported Katy Clark's bid to become deputy leader but despite massive support from some sections of the unions she appears destined to fail.

Relatively unknown until she played a starring role in the referendum campaign, Ms Dugdale has travelled the length and breadth of the country promising new politics, and a different way of doing business.

Her credentials on social justice bear scrutiny from all quarters. She has championed the living wage, called on the energy firms to freeze their bills, and backed a 50p tax for top earners. She will be well placed to take on all comers.

The battle for the Scottish vote is likely to focus on social justice. Labour will want to regain the mantle of the most socially progressive party. The SNP will have to decide whether their priority is social justice. And for those in Scotland who want to live in a socially progressive country, they must decide if it is Ed Milliband or David Cameron who will deliver it.