Ruth Davidson is the relatable leader the Tories so desperately need as PM – too bad she doesn't want the job

Today, Ruth Davidson made political history. It won’t qualify as her most significant. For the pregnant leader of the Scottish Conservatives – saviour of her party in a nation where the very word “Tory” had been an obscenity for decades – only becoming prime minister could do that. And this, Davidson tells The Sunday Times, will never happen.

The tiny slice of history lay in her dismissal of the rumour that she has a cunning plan to come south and replace Theresa May. “B*****ks”, she responded to that in what I believe is the first incidence of a British mainstream party leader using a swear word on the record.

A few inches across the same paper’s front page, meanwhile, another interviewee, Ms May, is quoted saying: “That choice of language is completely inappropriate. It’s not language I would have used.”

As it happens, the PM wasn’t talking about Davidson’s “b*****ks”. She was referring to Boris Johnson’s accusation that she’s strapped a suicide vest to the constitution with her Chequers proposals for Brexit.

But even if it is purely an accident of optics, the juxtaposing of such differing linguistic styles strikes a telling contrast. One of these female Conservative leaders is a warm and natural human being with the confidence to be wholly herself even when on duty. The other, without delving into the argot of robotics, is not.

If you’ve developed a suspicion (whatever your personal politics) that Davidson really, really ought to be the Tory national leader, nothing will cement it like the interview in which she says she never will be.

There may be another dollop of political history in the credibility of that declaration. While all politicians with a shot at the top job deny the ambition, until now every denial has needed seasoning with the contents of Siberia’s two largest salt mines.

Until the moment he challenged Ms Thatcher, Michael Heseltine used the formula “no conceivable circumstances”. Before he knifed Boris and ran for about three minutes himself, Michael Gove went further by offering to sign the disavowal in his own blood.

Ms Davidson would not talk so flippantly about the drawing of blood. She has a history of cutting herself, as she reveals in her new memoir Yes She Can: Why Women Own the Future, and as she confirmed to interviewer Decca Aitkenhead by lifting a sleeve.

Where Tony Blair theatrically claimed to have “scars on my back” from attempting public sector reform, Davidson has actual scars on her arms from self-harming with razors and glass during a drunken late adolescence plagued by suicidal thoughts.

The potential value of that admission, at a time when depression and self-harming are more epidemic than 20 years ago, is incalculable. It isn’t easy for anyone to confess to having suffered mental illness. For a political leader to speak openly about it, and of her fears about relapsing, is an epic display of personal courage.

The untold thousands of young people enduring the same horror will feel normalised if they read this. They may even find light shone on their darkness by the example of someone who doesn’t regard talking about it as an expression of weakness. Quite the reverse.

Davidson is professionally unusual, to put it mildly, in other ways. Traditionally, top level politicians save their craving for more time with the families they’ve neglected for years until their careers go into terminal decline.

She is doing it when she has a viable path to Downing Street, or at the very least a major cabinet job. She also has an IVF baby in her womb (one reason to love her is the empathetic refusal to crow about the process of it working first time for her and her wife, knowing most people are less lucky).

Being one of two mums, and not an absentee father figure, is her priority. “On a human level, the idea that I would have a child in Edinburgh and then immediately go down to London four days a week and leave it up here is offensive,” she says.

On a human level, Davidson towers above her leadership non-rivals. At the age when Boris was swanning about in his Bullingdon tails, casually signing cheques to proprietors of mildly wrecked Oxford restaurants, she was engaged in a more challenging battle than one deploying bread rolls as ballistic missiles.

Now, when he is so plainly willing to burn his country for power, she chooses family over ambition in a way that might be unfamiliar to him. It may have been a long shot for a devout Remainer to be elected to lead the Tories in this era. If the Stupid Party thrice rejected Ken Clarke, the last recognisably human working class Tory hero, perhaps it’s a stretch to imagine it plumping for her now.

But her wit, lack of pomposity and socially ultra-liberal voice would have hugely benefited the Tories from any Westminster role. She saved her party last summer with the miracle of the 13 Scottish seats that let May form a minority government. She might have saved it again, and more permanently, by redefining its national image.

Instead, she prefers to stay in Edinburgh (who in their right mind wouldn’t? It makes Paris look like Portsmouth), and live as normal and stable a life as the job allows.

“I value my relationship and my mental health too much for it,” she says of the premiership. “I will not be a candidate.” Jeremy Corbyn will be relieved that the Davidson paradox is plagiarised from the oeuvre of Joseph Heller. Anyone sane enough to be prime minister, this remarkable survivor of mental illness seems to be telling us, is too sane to want to be prime minister.