SCOTTISH Tory leader Ruth Davidson last week delivered one of the most withering slap downs heard in Holyrood for years.

After Nicola Sturgeon accused her of “cosying up” to Boris Johnson, whom she had disdained in the past, Davidson threw a dart at her with Alex Salmond’s face on it.

"I've never had a problem standing up to the alpha males in my own party. I wonder if the First Minister is able to say the same?" Davidson said.

It was a brutal retort that even Malcolm Tucker, the foul-mouthed spinner in The Thick of It, would have refrained from using. Sturgeon looked dazed.

However, while Davidson emerged as the winner from First Minister’s Questions, last week was probably her worst ever as party leader. And it was not her fault.

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With Theresa May finally setting a date for her departure, Boris Johnson is the favourite to become Tory leader and the next Prime Minister. Such an outcome would be a disaster for Davidson and hobble her chances of succeeding Sturgeon.

We do not need to speculate on what Davidson thinks about the former Foreign Secretary. Senior Scottish Tories last year embarked on a campaign, dubbed ‘Operation Arse’, to stop him taking over from May. Davidson effectively banned him from her party’s conference in Aberdeen.

Davidson’s allies fear her plan to become the next First Minister will be derailed if Johnson is the next PM. His lack of self-discipline and propensity for gaffes means she will be walking on eggshells, constantly wondering when next he will cause her trouble. Sturgeon will hold her culpable for his actions in office.

Her colleagues also worry about the impact a Johnson premiership will have on the Union, which has been the Scottish Tory meal ticket in recent elections.

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A senior Conservative told me there are three categories of Tory MP when it comes to the Union. The first is the “English nationalist” - think of Mark Francois or Peter Bone - whose commitment to Brexit is far stronger than a passing belief in Scotland remaining in the UK.

The second group - probably a majority of Tory MPs - is comprised of those who have never given much thought to Scottish politics but, when forced to take an interest, quickly become animated Unionists. May can be placed in this category.

The final type of Tory MP is someone who has an instinctive grasp of the Scottish constitutional dimension and does not require a crash course in the Barnett formula or the SNP. Michael Gove is the leader of this smaller pack.

Many Scottish Tories believe Johnson is in the first group. He has regularly denounced the Barnett Formula - the system that allocates public spending to Scotland - and has played the English grievance card on a range of sensitive cross-border subjects.

One of his few forays north of the border was in 2006, when he made an improbable and unimpressive bid to become Rector of Edinburgh University. Here is how he remembered the episode years later:

“I destroyed a massive poll lead by announcing that I was not only English but in favour of top-up fees, and ended up coming third and having beer poured over my head.”

Scottish Tories believe Johnson can make the jump from the first group to the second, but only with intensive remedial classes. Even if Johnson reinvents himself as a champion of Scotland’s interests, many doubt whether it will be seen as a sincere conversion.

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Sturgeon last week suggested that indyref2 would be justified by Johnson becoming PM. This is nonsense. Just as it would be ridiculous to equate Scotland with the Scottish Government, so too would it be unreasonable to argue that the UK Government is the same thing as the UK.

However, these must be worrying times for supporters of the Union. The pro-UK side defeated independence in 2014 by focusing on voters who were anxious about the economic risks of independence. For this cohort, the benefits of the Union were transactional, not emotional.

Five years later, the UK is on the verge of economic self-harm by virtue of some form of hard Brexit. A general election will likely offer voters a choice between a Marxist and a buffoon. Westminster is, at this moment in history, not seen as a vehicle for progressive change.

If Johnson wins, Scottish Tories fear the conditions will exist for an increase in support for independence and a second referendum. Sturgeon was scowling last week, but it is Davidson who may be grimacing in the longer term.