The late Tony Benn used to describe politicians as either “signposts” or “weathervanes”.

Signposts, he argued, signalled the way ahead, standing firm no matter how great the criticism, while weathervanes tended to spin on their axis, changing, if you like, with the weather.

In Paisley and Renfrewshire South MP Mhairi Black’s rightly celebrated maiden speech she deployed this quote from Mr Benn to make a tribally political point, presenting the SNP as a political signpost and Labour as an ever-spinning weathervane.

This is self evidently a deeply held view within the SNP; indeed Nicola Sturgeon has made the point herself, firmly in the belief that her party has kept its head while all about it others lose theirs.

But Nationalists are in danger of believing their own hype; indeed there’s an increasing sense of hubris, layered on top of referendum-era sanctimony, about the SNP and in particular “the 56” at Westminster.

I’m not the first to have detected this increasing self-satisfaction. The Times columnist Hugo Rifkind called it “priggishness”, a mission “not just political but moral, too”, while in the Guardian Deborah Orr praised Miss Black’s maiden speech but observed (rightly) that rhetoric didn’t “feed hungry old men or young children”.

Of course this duo could be dismissed as “Unionist commentators”; they would say that, wouldn’t they? There are critics on the other side, too. Take Kevin McKenna in yesterday’s Observer: “The 56”, he wrote, are “intoxicated on their own conceit”. Andrew Tickell in Scotland on Sunday, meanwhile, despaired of the SNP continuing to flog a dead horse (Labour) but failing to articulate what they are for rather than what they are against.

This, I’m increasingly convinced, is perfectly deliberate. In Drew Weston’s book, The Political Brain, he demonstrated how Democratic candidates in the US generally deployed arguments of reason, marshalling statistics and policy detail, while Republicans appealed to moral instinct and gut feeling. And in doing so Republicans generally won, for the political brain is primarily an emotional one.

During the referendum campaign the SNP identified that reality and exploited it relentlessly, as they did again during the General Election campaign, appealing to many Scots’ sense of moral superiority. Now this is self-evidently an effective way of winning elections, but a complete distraction from the business of government. But then the SNP realise that, if they allow themselves to be judged purely on their record in devolved government, they'll likely be found wanting.

This takes me back to Mhairi Black’s political hero, Tony Benn. A Labour right-winger in the 1960s and then a born-again Marxist in the 1970s, he did not practise what he preached. Nor do the SNP for, despite their resolute belief in their own consistency, Nationalists (like most other politicians) have shifted constantly over the past few decades, ideologically, tactically and even constitutionally.

Take English Votes for English Laws (Evel), suddenly opposed on a point of “principle” by “the 56” at Westminster. The Conservatives have advocated some sort of Evel at every election since 2005, while the SNP have changed their mind twice since the end of last year. Initially they were all in favour, chastising Labour for not agreeing with their self-denying ordinance, then Ms Sturgeon made it clear her MPs would vote on English issues with a financial implication (fair enough), and now, we’re informed, even that qualification won’t apply.

Yet curiously it’s the SNP that emerge from this series of volte-faces as “principled” and consistent while the wicked Tories, as usual, are condemned as opportunistic. “The 56” are also taking a Year Zero approach to the recent election, depicting themselves as the first authentic representatives of Scotland in the House of Commons, demonstrating extraordinary contempt for generations of Liberals, Labourites and, yes, even Conservatives who have ably represented nation and constituents for decades.

The identification with Tony Benn also reveals another Nationalist conceit, that theirs is a left-wing party that has never pandered to reactionary political forces, unlike Labour. Indeed, Ms Black and several other new SNP MPs railed against “neo-liberalism” in their maiden speeches, seemingly oblivious about their own party buying into that economic orthodoxy long ago.

Interviewed by Prospect shortly after her election, the SNP MP Tasmina Ahmed-Sheikh admitted her party’s economic stance was a mix of pro-business ideas which would “traditionally be thought of as centre or centre right”, albeit with a strong sense of social responsibility. But when challenged that she was, therefore, a Blairite, she sounded horrified, replying: “Absolutely not.”

She went on to suggest that the SNP’s recent election victory had shifted politics away from the traditional axis of left and right, which was quite a claim. Meanwhile, Alex Salmond has been talking up an alliance with a Jeremy Corbyn-led Labour Party for, of course, the guardians of the flame have deemed the Member for Islington North ideologically pure (though even the former First Minister acknowledged that “Jeremy might want a bit less of the private sector in the economy that I would”).

In Diarmaid Ferriter’s new book, A Nation and Not a Rabble, he notes that during Ireland’s “revolutionary period” some viewed nationalism as “about will and spirit and antiquity, an appeal to the dead generations”, while for others “it was something that needed to be called into being and could include social aims”. The same split exists in today’s SNP, with Dr Paul Monaghan – whose maiden speech burned with righteous indignation about the Highland Clearances – representing the former and Mhairi Black (who boldly claimed that “nationalism has nothing to do with what happened in Scotland”) the latter.

Other SNP maiden speeches have made high-faluting references to Rosa Parks, Franklin D. Roosevelt and Keir Hardie, another constant refrain being “we didn’t leave Labour, Labour left us”. Ms Black’s maiden speech, commented group leader Angus Robertson, was “principled and passionate”, but while no one could question the latter adjective, can a brand new 20-year-old MP really be called “principled”?

Giddy on their own brilliance, meanwhile, the SNP operate in an unprincipled way while presenting every U-turn or tactical shift as a point of principle. Fox hunting, for example, was a hitherto unknown point of principle, while on fracking – not to mention a range of other policy areas – the party increasingly faces both ways.

History, as ever, repeats. In the 1980s and 90s it was Labour who were holier than thou, demonising their opponents’ ideology while gradually adopted their central tenets. Thus Labour eventually became what they claimed to hate (the Tories), and the SNP are falling into the same trap: dissing the “Red Tories” while becoming virtually indistinguishable from them; that is, a rhetorically “radical” party terrified of offending middle-class voters.

And they liked Tony Benn because he, like them, increasingly sees the world in black and white, full of problems created by lesser politicians but with easy, cost-free solutions. Next year I’d love to see the SNP fight the Holyrood election on a Bennite prospectus, advocating withdrawal from the European Union, radical redistribution of wealth and state control of the economy.

But, of course, they won’t because they know they’d lose. Better to preach from the moral high ground than actually risk upsetting anyone.