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The Scottish Conservative (and Unionist) party has enjoyed something of resurgence in recent years under the energetic leadership of Ruth Davidson.

The party's opponents are convinced that Boris Johnson as prime minister could put an end to all that - and could even put an end to the union between Scotland the rest of the UK.

It's true that Mr Johnson could hardly be more different from the down to earth, plain speaking and Remain-voting Ms Davidson. The two are not friends and have vehemently disagreed before.

Some observers like to speculate that Boris will appear to Scottish voters to be the very epitome of the upper-class English ruling caste that Scots so dislike. But policy may well prove to be more important than personality.

Mr Johnson appeared to have a weak grasp of the dynamics of devolution when he proposed tax changes that take no account of the fact that income tax rates in Scotland are set by the Scottish Parliament.

But since then he has promised Scottish Tory MPs he will set up a "union unit" inside No 10 to check every policy. If he knows what he doesn't know, then maybe he can avoid these gaffes as PM.

It's Brexit that may be his undoing, in so many ways. In Remain-voting Scotland, his problem is that the idea of a no deal Brexit is far less palatable than it is in the rest of the UK. The harder the Brexit Boris delivers, the more the Tory party in Scotland could suffer.

If Prime Minister Johnson [he will officially take on the role this Wednesday] pursues a Brexit policy at odds with what most voters in Scotland would like to see, then it's possible they may change their minds about whether remaining part of the UK is in their best interests. Some recent polling evidence suggests as many as 60% of voters could vote "yes" to independence if we leave the EU with no deal.

In the end it may not be the precise details of any Brexit deal that stokes desire for independence - or indeed the character of any individual politician - but a sense that Scotland has different aspirations from the rest of the UK, which can't be reconciled within the current union.

If Mr Johnson wants to keep the kingdom united, he will need to take care not fan those flames.