Nicola Sturgeon’s decision to bring the Scottish Greens into government is the surest sign yet that, despite almost securing an overall majority in May’s elections, the Nationalist regime in Edinburgh is in trouble.

Because why do it? The Greens have served as loyal foederati to the SNP for years. There was some theatre of brinkmanship ahead of key votes, occasionally, but the smaller separatist party always fell into line behind the larger.

Officially, this is about creating a Scottish Government with a Holyrood majority for another independence referendum, which will thus increase the pressure on the UK Government to grant another vote.

But this isn’t especially plausible. It isn’t obvious why Boris Johnson will take the existing separatist majority more seriously on the basis that its two parties are put their working arrangement on a slightly more formal footing. He is under no pressure to sign off the one thing that would pull the fraying Scottish nationalist movement together, not to mention likely consume the rest of his premiership.

If that nominal gain is illusory, the costs to Nicola Sturgeon are real. Even commentators minded to give her a sympathetic hearing have been quite clear that many of the Greens are cranks. By giving them a formal stamp of approval by bringing them into office, it will be much harder to distance herself from their views on issues such as the economy which will be central to winning over voters sceptical about independence.

The First Minister, for all her flaws, is an extremely capable politician and will know all this. So if she doesn’t need a formal agreement to legislate, there must be some other motivation for taking the risk. I suggest it is sharing the blame.

Most pro-independence voters are at least centre-left. This means that if the SNP starts to lose support over the next few years, the Greens are the natural home for any defectors who aren’t prepared to switch to a unionist party such as Labour. (The much smaller share of right-wing nationalists have Alex Salmond and his Alba Party.)

But if the Greens are part of the Scottish Government, that both restricts their freedom of manoeuvre and makes them culpable for its failings, on everything from management of public services to securing a second independence referendum. Not only might this stem potential defections amongst the voters, but it could even see Green voters go SNP as the distance between the two parties narrows further and their separate appeal is eroded. As Angela Merkel reportedly said of coalitions, its the little party that gets smashed.

If this is the reasoning, it means that Sturgeon is making moves which hinder her efforts to reach out to swing and pro-UK voters in order to shore up the SNP’s dominant position within the existing separatist electorate. This would be of a piece with her party allegedly spending a ring-fenced referendum fighting fund on office improvements, and strongly suggests the Nationalists don’t think a second referendum is at all imminent.