Displayed in the main reception room of Bute House is Sir James Gunn's painting of the former Secretary of State for Scotland, Thomas Johnston.

It's a fine portrait, depicting Johnston in one of his characteristic double-breasted suits and bow ties. He has the modest demeanour worthy of a fine politician.

It would have been in eyeshot of the First Minister as he made his elegantly crafted (and movingly delivered) resignation statement on Friday afternoon, an historic occasion I witnessed on television along with every other proscribed journalist.

I suspect Alex Salmond fancied he had an affinity with a notable politician now sadly forgotten by all but the politically obsessed. At the same time, the Kirkintilloch-born Johnston provides a useful prism through which to assess the SNP leader's long career.

A lot depends on how a politician is judged. If, as the modern era dictates, it is on the basis of presentational skill, tactics and strategy then Mr Salmond is up there with the greatest: a better strategist than David Cameron, a more skillful triangulator than Tony Blair and perhaps the best framer of political narrative since Margaret Thatcher.

Take his long-standing mission to displace Labour as Scotland's dominant centre-left party. Of late this has been presented as a point of principle, with the SNP holding true to the socialist spirit of Johnston et al while Labour has marched inexorably to the right, gradually shedding its soul en route.

That has been a triumph of style over substance. Why, then, was Mr Salmond so keen on displacing Labour in the late 1970s and early 1980s when the People's Party was at its most "authentically" left wing? And why, on becoming SNP leader in 1990, did he still have the same objective?

The truth, of course, is that he has always been a deeply tribal politician: he hated Labour because they occupied the position he sought for himself and his party.

The added irony was that, while deriding (New) Labour for moving onto centre-right territory, Mr Salmond was busy taking the SNP on a very similar ideological journey, embracing free-market economics and attempting to woo big business with his own prawn- cocktail offensive. Indeed all the evidence suggests he took this rather more seriously than Mr Blair - even Tone didn't make speeches lauding the Laffer Curve.

His economic analysis, meanwhile, was usually astonishingly thin. Mr Salmond's response to the global economic crash was a case in point - there wasn't one; rather than engaging with its underlying causes and fashioning a coherent critique he simply assigned blame (he was always adept at playing that particular game) to London and a regulatory regime he had hitherto supported. Growth would soon return, he argued, helped along by further tax cuts.

And whatever their considerable faults, at least the governments of Mr Blair and Gordon Brown managed to reduce child and pensioner poverty while redistributing wealth via tax credits, tangible social achievements without real parallel in Mr Salmond-led administrations. Given the considerable autonomy enjoyed by the First Minister since 2007 he could, if he'd desired, have begun a significant assault on inequality via income tax, property rates and education.

Instead he opted to talk big, contriving public policy in narrowly tactical terms, convincing those on the Left his hands were tied by a limited devolution settlement and hiding behind pseudo-progressive policies that actually featherbedded the middle classes. Meanwhile he diligently cultivated vested interests, be they corporate or media, while presenting himself as a figure outwith the Establishment.

Remarkably, he managed to present himself as "different" when in fact he was every bit as cynical and opportunistic as anyone in the "Westminster elite".

But those willing to sign up to his big idea were willing to forgive a multitude of obvious sins, for as First Minister he appeared subject to different rules; the cynicism (rightly) afforded to other politicians was completely suspended when it came to constant promises of jam tomorrow. Again, his followers were prepared to believe that he was an exception to the rule; he - unlike everyone else - would actually deliver.

Part of the problem was that he never fully abandoned bad habits picked up in opposition. Long accustomed to punching above his party's (once insignificant) weight by saying outrageous things and constantly shifting position, in the half-way house between government and opposition, he simply carried on, indulging in intellectual contortions that would have seen any Westminster politician laughed out of Parliament. He turned riding two horses at once into an art form.

History was also merrily rewritten in a way that would have appalled Tom Johnston, a noted chronicler of the Scottish working classes.

On Friday night I heard one of Mr Salmond's former advisers attribute the establishment of the Scottish Parliament to his former boss, which is curious given he'd led a party formally opposed to devolution. Only in 1997, faced with a referendum, did he jump aboard the devolutionary bandwagon.

He was at it again yesterday morning, claiming that voters had been "tricked" into rejecting independence by a late promise of more devolved powers. Beyond the obvious point that the "more powers" pledge had been before the electorate for almost a year, if an escalated timescale for delivery constituted trickery, where exactly did that leave the thousands of Yes votes delivered on the basis of blatant scaremongering about the potential privatisation of the NHS?

But then blatant hypocrisy never seemed to bother Mr Salmond. The Liberal Democrats, another party which wasn't spared his tribal warfare, were pilloried for reneging on their no-tuition-fees promise after the 2010 General Election, yet three years previously Mr Salmond had ditched a manifesto pledge to eradicate all student debt, even though it had arguably captured a significant chunk of the student vote.

And in spite of lofty rhetoric about being "positive", divide and rule was a hallmark of his style, as was phoney outrage.

Anyone not perceived as a threat was treated with charm and thoughtfulness, but for those who fell outside that category condescension, pettiness and often downright rudeness were the order of the day.

I can think of no other politician who behaved as badly as often and, more or less, got away with it.

All of this appeared to be forgivable if one believed in independence as an end in itself - and of course plenty did. In that context, Enoch Powell's maxim about all political careers ending in failure doesn't actually apply to Mr Salmond: given all the inconsistencies inherent in the pro-independence case, getting 1.6 million voters on side was a significant personal achievement.

I say personal, because the outgoing First Minister was central to the whole project, no matter how keen some have been to disassociate him from it.

A cursory glance at recent history demonstrates how much more credible the "dream" has become as a result of the Salmond magic, but at the same time his career is best understood as that of a salesman, and of course they aren't renowned for either consistency or intellectual integrity.

If only portraits could talk. Tom Johnston's assessment of his contemporary admirer would be intriguing.