We’ve been hearing the Union is in peril for a while now.

In fact, it’s almost become a cliche of modern Scottish politics.

The resignation of Jackson Carlaw is just the latest manifestation of that unfolding crisis.

The Scottish Tory leader’s decision to quit on Thursday took the Holyrood bubble completely by surprise.

Mr Carlaw said he had reached the conclusion he is “not, in the present circumstances, the person best placed to lead” the pro-UK case ahead of next year’s Holyrood election.

Internal polling was not good, party sources told journalists.

Fair enough; politics is a brutal business and no one knows that better than politicians.

There’s no doubt Mr Carlaw failed to make a dent in the SNP’s dominance.

But getting rid of a leader in the middle of a pandemic, just months away from a crucial election, is a bold move, to put it mildly.

Of course, ditching a dud for a fresh face is all very well if you’ve got a sparky hopeful waiting in the wings. But who fits the bill?

Douglas Ross, the MP for Moray since 2017, hopes he does. He announced his leadership bid yesterday and looks set to run unopposed.

The former Scotland Office minister recently made the headlines when he resigned over the Dominic Cummings scandal.

If successful, he will seek election as an MSP at the next Holyrood election in May. He has asked former leader Ruth Davidson to handle First Minister’s Questions until then.

Ms Davidson was once the bright future of her party and still has a rare, natural ability to connect with people.

It remains to be seen whether Mr Ross has what it takes.

But the Scottish Tories can’t escape their association with the party in Westminster and Prime Minister Boris Johnson, who is unpopular north of the Border.

They are not the only unionist voice in Scotland, of course.

Scottish Labour has tied itself in knots in recent years but leader Richard Leonard recently reiterated its opposition to a second independence referendum.

The problem is he’s been in charge since 2017 and has failed to cut through with the public.

The party’s downfall is a familiar story and can’t be pinned on any one leader, but recent results have been truly woeful.

As polling guru Sir John Curtice pointed out during an interview on the BBC yesterday, it’s not so much that the Tories have lost their own support.

It’s more that Scottish Labour’s weak position means the Tories need to do much better if they want to stop the SNP securing a majority at next year’s Holyrood election.

Mr Ross and Ms Davidson will have just nine months to turn things around.

The words of Adam Tomkins, a senior Tory MSP who is standing down in May, will be ringing in their ears.

“For the first time in Scottish history, independence now looks like it might not be the minority pursuit that it’s always been but the position of a majority of Scots,” he told the BBC.

The 2021 Holyrood election is the SNP’s to lose.

And despite what Mr Johnson says, if the party wins an overall majority on a clear manifesto commitment to hold a second referendum, it’s hard to see Downing Street blocking this.

Basic democracy would demand it takes place.

This is implicit in Mr Carlaw’s resignation, and his “painful conclusion” that he is not the best person to lead the fight for the Union.

At this rate, the only thing the independence movement has to worry about is itself.

To quote Prof Curtice again, the “brutal truth...is that the biggest risk the SNP face to the realisation of their ambitions are their own internal divisions”.

These came to the fore again only yesterday, when Joanna Cherry, who is seen as close to former first minister Alex Salmond, quit the contest for Holyrood next year.

The SNP MP expressed her anger after a rule change by the party’s governing body meant she would have to resign her Westminster seat before standing to be an MSP.

“This move opens a serious political divide inside the SNP and virtually assures the emergence of other pro-indy list parties at the Holyrood election,” wrote former SNP MP George Kerevan on Twitter.

But with the opposition struggling and the SNP on course for a landslide, it’s hard to see the argument for new pro-indy groups.

Then there’s the Alex Salmond affair, both the looming Holyrood inquiry and the wider fallout.

Former SNP deputy leader Jim Sillars has already suggested a book Mr Salmond is writing about his case will be like a “volcanic eruption” for the party.

It would be a peculiar fate indeed if the independence movement engineered its own downfall at a time when the Union has never seemed so weak.

But politics is carried out by people, and there’s nowt so queer as folk.