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The Scottish Conservative leader insisted she would give the same consideration to endorsing the former foreign secretary as other candidates for the top job. It came as Ms Davidson made an impassioned plea to Scots to “stand up against” Nicola Sturgeon’s crusade for independence in tomorrow’s European elections. She said the First Minister will use any vote cast for the SNP as a mandate for the bid to break up Britain.

Ms Davidson also acknowledged voters’ resignation, frustration and disbelief over the delay to Brexit. And she issued a renewed call for MPs to find compromise over Theresa May’s troubled deal. In a wide-ranging interview with the Scottish Daily Express, Ms Davidson accepted Nigel Farage’s Brexit Party will do “very well” tomorrow. But she played down his prospects of future electoral success and rejected suggestions of a Tory pact with Mr Farage on delivering Brexit. Opinion polls have suggested the Scottish Tories could lose their sole MEP as voters defect to the former Ukip leader’s recently formed outfit. But speaking yesterday, Ms Davidson said the party’s “core vote” appeared to be “holding up reasonably well”. She also argued the forthcoming Tory leadership contest was a chance to “show people there is something beyond Brexit”. Ms Davidson made clear she will measure each candidate against three tests – strengthening the Union, advancing Scotland’s interests, and healing the divisions of the 2014 independence referendum and 2016 poll on EU membership. She is also pushing for candidates to back a devolution “delivery unit” at the heart of the UK Government. Ms Davidson has been one of Mr Johnson’s most persistent Tory critics and recently barred him from the Scottish Conservative conference.

Ruth Davidson accepted Nigel Farage’s Brexit Party will do “very well” tomorrow

Boris Johnson is leading the race to become Prime Minister

Asked if she could work with him if he wins the race, she said: “I have worked with him when he was Foreign Secretary. I will work with whoever the Prime Minister is.” She also quipped: “I haven’t had a phone call yet to ask me to run his campaign in Scotland. I am not expecting the call. “But I will genuinely judge him on the same criteria as I judge any of the candidates.” Pressed on whether she thought a Remainer or Brexiteer Tory was more likely to unite the country, Ms Davidson insisted she wanted to see as many candidates as possible. She suggested Mrs May’s coronation in 2016 when Andrea Leadsom pulled out was to blame for some of the party’s “issues” since then. “There wasn’t the testing of them both under pressure, under campaign conditions, and all of the things that your learn from that,” she said. Ms Davidson has spent the past week on the Euro election campaign trail to be met by “resignation and frustration rather than anger” on doorsteps over Brexit. She said: “There is also quite a bit of disbelief. People don’t really understand. ‘Well, you said we were leaving, so why haven’t we gone yet?’” But she argued MPs had failed to make a decision not “out of ­badness” but rather because of “sincerely held views”. She repeated calls from Tory and Labour benches to compromise and “start walking back to the middle”.

Brexit Party leader Nigel Farage