The professor of public law at Glasgow University is sporting a wristband from the pro-union campaign Vote No Borders, while his office in the university's law school is adorned with a "No thanks" postcard and large Team GB union flag.
On his website he describes himself as one of the leading constitutional law scholars in the UK and he's certainly one of the leading voices arguing the union's case on the internet. He does so under his own name on Twitter and in various blogs, and he is the force behind Notes From North Britain, the website which bears the tagline "Confessions of a Justified Unionist".
That said, the pro-union space on the internet is not exactly crowded. There are no pro-union campaigns on the web to rival those of independence supporters such as Wings over Scotland.
Tomkins had 3385 followers on Twitter yesterday morning. Wings Over Scotland had 15,200, and fellow independence supporter Bella Caledonia had 16,300.
Tomkins says he could have decided not to take part in the referendum debate, a decision he describes as perfectly valid. Instead, he declared as a No voter early on in the debate as he did not want to "just be an observer".
"I decided I cared so much about this particular issue I was not going to approach it from the position of independent neutrality," he explains. "Although I hope I have been objective, fair and accurate in my assessment of the legal issues.
"I am not a partisan, in the sense I don't toe anybody's line."
Tomkins has been involved in various aspects of the independence debate, including advising the UK Government on legal issues surrounding independence as part of an informal group of lawyers put together by Advocate General Jim Wallace. He was one of two independent advisers to the Strathclyde Commission - the Conservative review of how Scottish devolution should work - and has written a series of blogs for Vote No Borders tackling topics such as such as the legal and political "realities" of what independence would mean.
But his views on the issue have a personal basis. Tomkins was born in England and spent the first 33 years of his life south of the Border, before moving to Scotland in 2003.
"I am English and British, but I live in Scotland," he said. "My wife is Jewish and American, but lives in Britain as she would see it. My kids have dual US and UK nationality and they are Jewish: so multiple identities feel natural and normal.
"For me, that is what the independence referendum is all about - it is forcing me to choose, would I want to stay in an independent Scotland as a no supporter? I really don't want to have to choose between staying in an independent Scotland and returning to the much diminished rump of the UK."
When it comes to the Yes campaign, he is particularly critical of what he terms the "de-risking" approach by the SNP.
"I absolutely understand why the SNP leadership have played it in the way they have, as there is a natural minority in Scotland in favour of independence," he says. "They need to turn that natural minority into a one-day majority. That means they need people to vote yes who have not historically supported independence and who have not historically been SNP supporters.
"The mood music of the SNP campaign has been ... there is very little at risk here and not much will change. We will keep the Queen, we will keep the pound, we will keep the social union with the rest of the UK, we will still be in Nato, we will still be in the EU, we will still be in the UN."
But Tomkins says the problem with this approach is that it assumes that everyone will be willing to act in Scotland's interests.
"That is unrealistic, as everyone else will act in their own interests," he argues. "Just because something is in the Scottish national interest, doesn't mean it is in the interest of all of the people an independent Scotland is going to have to negotiate with. These negotiations are going to be really tough."
So far, so predictable. Where Tomkins departs from the Better Together script is in his predictions for the outcome of the referendum ... and in the outcome he believes would be best for Scotland.
Although he insists says there has not been a "credible" poll which has put the Yes vote about 45% and the No vote below 55%, he says that it is "not in the bag" and the Yes vote could yet win.
"This is not over - it doesn't feel over, it feels that this is something worth arguing about and is not a foregone conclusion," he says.
"However, I think the polls are probably right - the last few I have seen have put the No vote between 55-61%, it will be around about there."
And his best result for Scotland? A "narrow-ish" win for the No campaign - an outcome he argues would trigger much-needed discussion where devolution should go.
Tomkins says devolving income tax to the Scottish Parliament would transform politics in the country by triggering a "grown-up" argument about tax and spend. He would like to see unionists and nationalists work together to develop devolution further, arguing there has been a "silo" approach to constitutional politics for too long.
"The independence referendum has been divisive - it is necessarily divisive because it is a very emotive issue and because it is a binary question of yes or no - so it is necessarily polarising," he says.
"Once we have moved on from that polarising nature of the referendum, we need to move on to something we have never had - an all-party conversation about where we take Scotland's constitution next. That is what I want to see."