Len McCluskey this week praised Nicola Sturgeon and said that Labour should be prepared to work with the SNP. He isn’t the only English lefty to fall for the charms of Scotland’s First Minister. ‘Can I vote SNP?’ was one of the most popular Google searches among non-Scottish voters who watched the main leaders’ debate at the start of the campaign. The effect has been similar to Cleggmania in 2010 with a rival party leader appearing to say the sorts of things many Labour supporters have longed to hear from one of their own. Why shouldn’t they be attracted to the idea of Labour forming a progressive alliance with the SNP, Plaid Cymru, SDLP and possibly a Green or two? For very good reasons, as it happens.

The case against a deal with the SNP is usually made by Labour tribalists and couched in terms so self-serving as to be immediately discounted by its intended audience. The party might do better if it showed more self-awareness by acknowledging that there are valid reasons for people in Scotland to feel angry with it.

The argument for steering clear of the SNP isn’t that Labour has a monopoly of progressive virtue. It clearly doesn’t. The real reason for being a Nat-sceptic is that, aside from nationalism, the SNP has no ideological core of its own and simply instrumentalises progressive ideas to advance the regressive goal of separatism. For the non-Scottish left there can be no question of a principled and trusting relationship with the SNP because you can’t build a common project for social change with someone whose first and only purpose is to smash up the political community to which you both belong. The left in England and Wales may want the UK to work differently, but they definitely want it to work. Nicola Sturgeon and her party want it to fail.

The SNP could have proved otherwise by refocusing its priorities on areas of shared interest with the rest of Britain when it lost the referendum, but it spurned the opportunity. Like true vanguardists, the self-styled ‘45’ decided to set democracy and majority opinion aside and behave as if they were real voice of Scotland. Their pledge that the referendum would be a “once in a generation” event was immediately ditched in a frenzy of debate about how soon a rerun could be engineered and what ruses would be needed to secure a different outcome. Everything the SNP does is now framed with that solitary objective in mind.

The effect has been to foster a dominant attitude that is highly sectarian and trending towards totalitarian. There is only one truth and one way to be authentically Scottish – the nationalist way. Anyone who disagrees with this is, as one SNP parliamentary candidate put it, the moral equivalent of a Nazi collaborator. There is no space for pluralism and honest compromise with a movement in this state of mind. The normal rules of democratic conduct don’t apply because it answers to destiny alone. When Nicola Sturgeon says that she wants to help the Labour Party, she does so in the same spirit that Lenin once advised his British followers support the Labour Party of Arthur Henderson: “as the rope supports a hanged man”.

The SNP’s progressive credentials don’t, in any case, stand up to serious scrutiny. When Sturgeon was asked at her manifesto launch to name a redistributive policy enacted by the SNP in Holyrood, she was unable to cite a single example. There has been plenty of middle class welfarism, but no effective measures to reduce inequality or poverty. Indeed, the SNP in power has resembled nothing as much as New Labour in its pomp, combining the worst reflexes of authoritarian statism and market liberalism with a superior, “we know best” attitude that brooks no opposition.

With the creation of a single national police force, the routine use of armed response units, a stop and search rate four times higher than the rest of the UK and plans to create an integrated ID database, the SNP has strayed into areas that even Tony Blair’s Home Secretaries backed away from. A new ‘named person’ law will create an army of state employed snoopers with a right to pry into the affairs of every family. The party has also taken a lurch towards democratic centralism with a new gagging rule that obliges its MPs to "accept that no member shall within or outwith the parliament publicly criticise a group decision, policy or another member of the group".

The SNP’s ‘business friendly’ approach of sucking up to powerful tycoons like Donald Trump, Brian Souter and Rupert Murdoch is scarcely any better then Blair’s cloying embrace of the super-rich, and arguably worse. The party’s flagship post-independence economic policy of attracting multinational companies by slashing corporation tax and undercutting the welfare budgets of other countries is the sort of tax piracy beloved of the neo-liberal right. The SNP’s claims to be anti-austerity have been revealed as baseless. Only opposition to Trident sets it apart; hardly an act of principle given that an independent Scotland wouldn’t be able to afford nuclear weapons.

At a time when Britain is crying out for a politics of the common good, the SNP stands for a militant politics of sectional advantage. It is rapidly acquiring the characteristics of a political religion, a faith-based movement that vilifies unbelievers and subordinates all other considerations to the attainment of national ‘rapture’ through independence. This sets it apart from other parties, even Plaid Cymru which takes a more pragmatic approach to independence and has already worked as part of a coalition in Cardiff. There are good reasons for people on the left to want a new kind of pluralist politics, but it’s no use pretending that you can pursue that vision with people who aren’t pluralists. Short of a second referendum defeat, the SNP is likely to remain a belligerent and destructive force in British politics. Progressives beware.