MANY of my correspondents regard me as a paid-up member of the Scottish National Party – or more usually a “nasty Nat”. In fact, I've never been a member of the SNP and the only party I was ever involved with was Labour – many years ago. I fell out of Nicola Sturgeon's good books when I criticised the Named Person scheme and the abandonment of the 50p tax band.

But I'd have to concede that the SNP is the most dynamic and effective political force in Scotland right now. An illustration of that is the quality of the candidates for the SNP deputy leadership, the race for which is now getting under way. At least three of the four candidates would be serious figures in any political party. Which is just as well, because this is arguably the most important deputy leadership race in the party's history.

The front runner is Angus Robertson MP, the SNP parliamentary leader who has been a powerful figure in Westminster. His interventions at Prime Minister's Questions have shown him to be not just a party leader, but at times the effective leader of the opposition, given Labour’s disarray. Indeed, on issues like the war in Syria, working tax credits, the Human Rights Act and on Trident he has managed to speak for the entire UK not just Scotland.

When Alex Salmond returned to his old haunts on the green benches after the 2015 General Election, many commentators assumed he would overshadow Mr Robertson, but there was never any chance of that. While Mr Salmond got up the noses of the establishment, Mr Robertson speaks to them on their own terms. That may be his handicap indeed, because at times he does seem sound like the establishment voice of the SNP. As defence spokesman he was responsible for ending the party's opposition to Nato, which almost split the SNP and led to resignations.

Also in the running is Alyn Smith .the SNP MEP who received a standing ovation for his “we didn’t let you down” speech to the European Parliament in June. He is an intellectual on the internationalist wing of the party – yes there is one – and is frank enough to admit that the party got it wrong on Europe during the Scottish independence referendum by assuming that the process of (re)joining the EU would have been automatic when it certainly would not have been. He is urging the SNP leader, Nicola Sturgeon, to take seriously the option of Scotland remaining in the EU even as the UK leaves. However, there is a strand of parochialism in the SNP and Mr Smith is not a parliamentary figure here.

Mind you, SNP membership has nearly quintupled since 2014 and no-one really knows what kind of party it is now. Someone who believes he does is the SNP MP for Edinburgh East, Tommy Sheppard. A former assistant secretary of the Labour Party in Scotland, and co-founder of the Stand comedy club chain, Mr Sheppard had been in the SNP for a matter of months before he won a seat that had been in Labour hands since 1924 in the 2015 tsunami. He gave a barnstorming maiden speech when he took up his seat in Westminster.

Mr Sheppard was a prominent figure in the Yes Scotland campaign and his pitch is very much to that constituency. He wants to transform the party structure and the way in which it develops policy. He says he has no problems with the current leadership of the party and isn't leading any left-wing alternative, but he's clearly expressing the frustration of many of the new members at the way policy-making is centralised in the SNP. That erupted at the 2015 party conference over fracking and land reform.

The fourth candidate in the race so far, Chris McEleny, also wants to put the grass roots in charge, though as leader of the opposition at Inverclyde Council, he is very much on the local authority wing of the party, which feels it has been rather eclipsed in recent years.

This deputy leadership election is important if only because the winner will be responsible for relaunching the independence campaign, which faltered after the resignation of Stewart Hosie following a messy affair in Westminster. With Nicola Sturgeon talking about a referendum as early as next year, people are wondering where the Yes Scotland campaign is. The SNP can’t win on its own. Party members have a crucial choice to make.