There is no escaping it. Nicola Sturgeon and Ruth Davidson have a mutual symbiotic relationship. Both benefit politically from the success of the other, and both could also lose from the decline of the other.

Davidson, by giving so much focus to her concern for the Union, requires Sturgeon to continue to bang on about a second independence referendum if she is to have any political relevance or any electoral appeal. Davidson claims to be Conservative and Unionist but after nine years at the helm of her Scottish party she has failed to develop enough distinct policies on education, health, housing and transport. At face value Davidson appears far more of a Unionist than she does a Conservative – to the extent of looking like a one-club golfer using her driver in the bunker and on the green as well as off the tee.

Ruth Davidson's stance on no-deal Brexit is helping SNP independence push – Ewen...

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This strategic error has become more noticeable to more people in her party, of late – and could be her undoing with the electorate and ultimately imperil the Union.

Sturgeon, by giving so much focus to her wish for a second independence referendum, requires Davidson to continue being a unionist Aunt Sally whom she can attack as London’s Conservative voice if she is to deflect attention away from the growing number of domestic policy failures she should be held to account for. Sturgeon claims to be putting education as her number one priority but last week’s fourth successive fall in Higher exam passes exposed that commitment as a sick joke. Sturgeon appears only interested in one thing: using anything to lay blame at Westminster so she can break up the United Kingdom, rather than demonstrate she is fit to govern a devolved administration and thus show she could make a success of independence. This strategic error is also becoming more noticeable to more people in her party – and will, ultimately, be her undoing with her party faithful if she does not deliver a second referendum.

For Davidson, her critical relationship with Prime Minister Boris Johnson is another problem brewing and it is entirely of her own making.

By backing three different opponents of Johnson in the Conservative leadership contest, she gave the appearance of wanting anyone but him, stoking up problems for the future. Instead she could have stayed above it all on the basis that she would have to work with the ultimate victor. It was another strategic error of judgment and it is already unnerving party members in Scotland who naturally want to see them working together successfully.

Davidson was perfectly entitled to articulate the case for remaining in the EU, and for seeking a soft Brexit while negotiations were taking place, but she has made a further strategic error in not supporting her leader in his pursuit of respecting the referendum outcome by taking the UK out of the EU on 31 October, “deal or no deal”. Johnson recognises the existential threat to his party if he fails to deliver on his word – and the crisis in confidence over our democratic processes that will follow if Brexit is delayed or a further referendum held.

Undermining the value of British democratic decisions undermines the value of the Union itself. Undermining her own party leader and Prime Minister imperils the Union more than anything Sturgeon can do at this time.

Davidson is wrong to argue that no one voted to leave without a deal. Many, including myself and others campaigning for a Leave vote, pointed out throughout the debates that on leaving the EU the UK should settle for trading under WTO terms until such time as a free trade agreement could be negotiated. It was hoped that this could be achieved within the two-year period following the triggering of Article 50, but it was never thought to be certain.

By deciding to negotiate a transition agreement first, without seeking to negotiate what trading arrangements we would be transitioning to, Theresa May, her chancellor Philip Hammond and, through her support, Davidson ensured that a free trade deal would never be offered. What we ended up with was a trap that would keep us in the European Union, paying hand over fist while having no say in the regulations we would in future have to accept – unless we sacrificed Northern Ireland’s place in the United Kingdom. Backing such a “deal” is by definition the antithesis of British Unionism.

May’s premiership is over, and with it her Withdrawal Agreement and its political declaration should be gone too. Even if a different party leader had been chosen there would have been a need for Davidson to take a fresh approach to leaving the EU. Davidson could have best helped, say, Jeremy Hunt, her party and the Union by starting to talk up the opportunities that Brexit offers. Instead she has chosen to repudiate leaving without a “deal” that threatens the Union.

By failing to define what her Scottish Conservatism means by offering attractive alternative policies to Sturgeon’s failure to deliver on the domestic front, Davidson raises questions about her conservatism.

By failing to repudiate May’s “deal” which would have ripped the Union asunder, and failing to talk up the advantages to Scotland of Brexit, by championing the return of fisheries management for Scotland; or talking up the opportunity for freeports at Prestwick, Hunterston, Grangemouth, Rosyth, Aberdeen and Dundee; or the benefits of new trade deals for Scottish businesses such as our distillers; or even the price advantages to Scottish suppliers and consumers that could come from the EU starting a tariff war, Davidson puts at risk her own MPs and raises questions about her unionism.

All it needs now is for Sturgeon to pivot towards domestic policy and show a willingness and ability to tackle Scotland’s domestic problems that devolution empowers her to do and Davidson will have the ground taken from under her. The symbiotic relationship will then be gone, and so too will Davidson’s attractiveness to the Scottish electorate.

It is time we saw the Conservative Unionist that Davidson claims to be, not the Liberal Unionist she resembles – telling us what taxes and regulations she will cut, what social and economic reforms she will champion and how she will make Brexit a success – because that’s what her Union voted for.

Brian Monteith MEP is chief whip of the Brexit Party

Brian Monteith: Ruth Davidson’s decisions have imperilled the Union