THE back story behind Professor Adam Tomkins becoming a Tory MSP sums up the rise of the Scottish Conservatives and the fall of Labour.

In the years before the independence referendum, Tomkins had no party allegiance but was carving out a reputation for himself as a robust defender of the Union.

Senior figures in Labour and the Tories were sniffing around and asked discreetly for advice on the constitution. The rest is history.

“I joined the Conservative party because I met two women: one, Ruth Davidson; and, two, Margaret Curran,” he says over lunch at Holyrood.

He explains: “Ruth knew that I wasn’t a Tory - and didn’t care. She wanted advice from people who she thought, rightly or wrongly, knew what they were talking about. Who I was, wasn’t as important to her as what I was saying.”

“Margaret was exactly the opposite. Every conversation we had, she would interrupt several times and say ‘you are one of us, aren’t you? You are one of us?’. She wasn’t very interested in what I was saying. She was just trying to figure out where I had come from and which tribe I belonged to.”

Labour’s “knee-jerk default tribalism”, as Tomkins puts it, was one of the reasons for joining the Tories in early 2014 and casting his first ever vote for the party in the same year. He became a Conservative MSP for Glasgow after the last Holyrood election.

However, there is an even more remarkable element to the Glasgow University professor’s belated embrace of centre-right politics.

When Tomkins moved to Glasgow from Oxford in 2003 - where he was the John Millar Chair of Law - he was a left-winger and critic of the monarchy.

He once attacked the “degrading rituals of pomp and servility that accompany majesty" and wrote: "You're either a monarchist or you're a democrat. You can't be both.”

In 2004, he made a speech on the same stage as SSP leader Colin Fox at an event to rival the Queen's official opening of the new Holyrood building.

He also co-wrote a republican tract with author Alasdair Gray, a partnership Tomkins recalls as being a “very intense, very odd period”.

Now 47, he says of his lefty past: “As far as the left-right question is concerned, yes, absolutely, I have been on a journey about that.”

Was he a socialist? “That’s a word I have always struggled with. I remember being as young as 15, talking to my vicar about whether we were socialists or not. I don’t know really.”

However, Tomkins insists he was always anti-independence: “One of the interesting things about that [Alasdair Gray] book was that we called it How We Should Rule Ourselves. But the ‘we’ for him was different from the ‘we’ for me. I was writing about Britain; he was writing about Scotland.”

How did it feel to vote Tory for the first time? “Like coming home,” he replies, a little sarcastically. And who did he previously vote for? “I’d floated around. Labour, Lib Dem.”

Other than Davidson’s pull, Tomkins says two policy issues convinced him he belonged in the Tory party: the welfare agenda of Iain Duncan Smith; and Michael Gove’s education reforms.

Educated at a comprehensive school in England, Tomkins says he would like his children to attend a state school in Glasgow, but is worried about classroom standards.

“I would very much like my kids to be comprehensively educated, but that is proving to be a bit of a challenge at the moment, because we can’t get them into the schools that we want to get them into. And so we are keeping our options open about that - and it’s stressful,” he says.

He is passionate about education, but sounds less convincing on the effects of modest tax changes pushed through by the SNP Government.

“I am not sure I would come here now, partly for economic reasons and partly for political reasons,” he says.

“The work I was doing in Oxford is the same work as I was doing in the University of Glasgow. Why would I do that same work in Glasgow, rather than Oxford, if I was going to be taxed more for it?”

Hold on. The Government decision to freeze the 40p income tax band means that people living in England are fractionally better off than those living north of the border. The difference, I suggest, is tiny: “These marginal differences are enough to swing balances one way or another.”

He adds: “Even the left-wing Adam Tomkins didn’t like paying tax.”

Given his shadow cabinet brief includes social security, Tomkins has faced criticism about the Tory Government’s “rape clause” policy.

After the Conservatives limited tax credits to two children, the Government relaxed the rule for women who had been raped, but only if the horrible detail was declared on a form.

Tomkins does not sound like a hardliner on the overall tax credits policy. “There are good arguments in favour of the proposition that child tax credits should be limited to the first two children in a family, but they are not so overwhelming as to be obvious,” he says.

On whether he would support Holyrood using its powers to reverse his own Government’s policy, he says: “If there is evidence that working families are being pushed into poverty, then that is an issue we would want to confront.”

As the Holyrood chamber beckons, one questions lingers. Is he still a constitutional republican?

“In the sense of believing that Parliaments must have the powers to hold their governments to account, yes. In the sense of the identity of the head of state, I’m really not bothered.”

If Labour had been shrewder, Tomkins might have been in Holyrood on the red side of the aisle. He would probably be their next leader.