Whether you love her or hate her, it will be a challenge to imagine Scottish public life without Nicola Sturgeon at the top of the Scottish National Party and the Scottish Government. She has been at or around the leadership of the former since devolution’s inception in 1999, and either Deputy First Minister or First Minister for over a decade.

However, just as Margaret Thatcher famously said (in relation to her number two, William Whitelaw) that every Prime Minister needs a Willie, Ms Sturgeon has needed a Swinney.

John Swinney has been arguably as important to the success of the SNP as Ms Sturgeon. As one of the "three S’s’" along with Alex Salmond, Mr Swinney can legitimately be seen as being an equal leg of the stool rather than an understudy.

Read more: Why Yes camp should meld the best of Yousaf and Forbes

He has, of course, been leader, and although that would not be his happiest time in politics, his role as the SNP Government’s Finance Secretary for the best part of 10 years was probably, quietly, his finest hour.

Only the most ideologically blinkered refuse to recognise the importance of a good relationship between the government and the business community, and Mr Swinney cultivated that relationship expertly. His easy style was combined with a gravitas that demanded the business community take him seriously. With genuine private sector experience, he knew the basic facts of life and was able to speak the language of economic growth.

He was equally reassuring in his role as Education Secretary, where one got a sense that he understood the need to promote excellence at the top at the same time as ensuring that those at the bottom were lifted (although, it must be conceded that the first of those has clearly not been achieved of late).

Yes, the success of the SNP and of the wider Yes movement has the fingerprints of Mr Swinney all over it. 

When he announced his resignation from the office of Deputy First Minister, shortly after his boss had done the same, it felt as though the era of Swinney was over. 

Read more: The choice for the SNP may be between party or country

However, I am not so sure. Indeed, in the event that Kate Forbes, the outsider, pulls off a win on Monday at the culmination of the SNP leadership contest, Mr Swinney’s most important role in politics may be ahead of him rather than behind him.

Twelve years ago, when Ruth Davidson won the contest to be leader of Scotland’s Tories, she commanded very little support from her MSP group. The leadership contest was fractious with a healthy dose of scepticism about the impartiality of the party establishment (ring any bells?), and there was a credible threat that Ms Davidson would struggle to assemble a shadow cabinet. 

Had it not been for David McLetchie, she may not have. Mr McLetchie, my much-missed first boss, was a giant within the party. He was respected by all sides and was, in heart and head, a party loyalist. He saw the party at risk, and he dealt with it, having the quiet chats that needed to be had to help Ms Davidson put a team together. 

It is little known and little remembered, but it legitimately changed the course of that party.

I see many parallels between that Tory contest and this SNP one. Ranging somewhere between a skirmish and a civil war, both contests provided an existential threat to the party concerned. And, like that Tory contest, this one may need the gigantic clout of a respected former leader to pick up the pieces and glue them back together.

A victory for Kate Forbes is likely to create a problem akin to the one Ms Davidson faced. Very many MSPs and Ministers have said very many things, in public and in private, which will need to be retracted in order for a Forbes-led party to pull itself together. 

This would be a change of gargantuan proportion – perhaps the biggest single event of the devolution era. It would involve, most likely, the dissolution of the agreement with the Greens, with the most interesting question being whether or not the jump comes before the push. It would almost certainly involve a pivot back towards the business community and a clear push towards economic growth being the key priority on which all other public policy is dependent. 

It would involve, at least in long-term intention if not short-term action, a wholesale re-design of the provision of healthcare, education and other public services.

Perhaps most importantly, it would involve an institutional reality check amongst the elected politicians, too many of whom have for years swallowed the line that independence is just around the corner when it is demonstrably and obviously nowhere near.

Read more: The humanity of Nicola Sturgeon will long stay with me

And, though it seems a distant memory for those of us observing the day-to-day leadership election, we should not underestimate the impact of Ms Forbes’ statements around gay marriage and the birth of children outside of wedlock on the very colleagues on whom she will depend to fill Ministerial posts and support Budgets. 

These people have been shocked, and in many cases personally hurt because their own personal circumstances meet precisely the criteria appraised in Ms Forbes’ religion-based answers.

A good number of MSPs will, in the event of her victory on Monday, decide not to help. Not to serve.

On these pages last Friday, I surmised that the best future for the SNP may be as two parties rather than one. A party of economic centrism and public sector reform (a Kate Forbes party), and a leftist party of tax and spend (a Humza Yousaf party). A refusal to serve under the leadership of Ms Forbes would undoubtedly push us towards this outcome.

And whilst I, as a commentator, may believe that to be a decent idea, it is very clear that those inside the SNP do not agree.

Enter John Swinney. If the one-party model is to be preserved under Ms Forbes’ leadership, any decision by those Forbes-sceptics to withdraw labour would need to be reversed. They would need to be convinced to serve for the greater good.

There has been much talk in the last few weeks of Ms Sturgeon’s legacy. The legacy of Mr Swinney may be made next week.

Andy Maciver is Founding Director of Message Matters and Zero Matters