Nicola Sturgeon might seem an odd choice to make the case for maintaining a political union under threat from separatists.

After all, just two years ago she was spearheading a campaign for Scotland to leave the UK, a constitutional compact of much longer standing than the current incarnation of the European Economic Community.

Indeed, some in her party spent 2014's referendum speaking about Westminster in terms that make Ukip's fulminations against Brussels seem tame.

The suave and urbane Times columnist Kenny Farquharson doesn't even think she should be taking part in tonight's EU debate on ITV.

I get why Kenny, as a Remainer, might dread the pitfalls of a Scottish Nationalist haranguing Middle England on how they ought to vote on June 23.

But when you stop to think about it, the First Minister is the perfect choice for the In campaign. Who better to argue against nationalism than a nationalist? Who is more familiar with the gripes and grievances, the dull obsessiveness and cloying self-pity? Sturgeon's nationalism is infinitely more considered than that of hard-right Brexiteers but there are overlaps in logic and it is there where the SNP leader can strike.

Yes, she can say, the Remain campaign has been a dismal, squalid little shriek of fear and pessimism. But what is Leave's Plan B if we do Brexit and their assumptions prove to be ill-founded? The Yes campaign in Scotland learned the hard way what happens when you have no credible back-up plan.

Hope is vital and enriching to political debate, she can tell the country, but the anti-Brussels nationalists aren't offering hope; they're playing to the basest of instincts about immigration.

Leave doesn't want Britain to make its own way in the world, she can point out; it wants us to withdraw into ourselves.

And of course the Brexiteers on the platform tonight will accuse her of hypocrisy. You want independence for Scotland, they'll dig, but think Britain should be ruled from Brussels.

The answer to that should be: The Union and the EU are not the same thing. Britain gets a better deal from the latter than Scotland gets from the former. Britain is an equal partner with an equal say in the European project. Scotland enjoys no such relationship with Westminster.

Her underlying message has to be: I know about trying to break away from a complex political and economic union. I have the scars to show for it and learned hard lessons along the way. Take it from me, this Leave campaign is amateur, clueless, and gambling with people's jobs.

And who is she up against? What fine specimen of Euroscepticism have they commissioned as their prosecutor of the Brexit case?

Alexander Boris de Pfeffel Johnson.

The best that can be said of him is that he knows his audience.

Writing in the Daily Telegraph on Tuesday, he warned that Remain voters, if victorious, would "have helped to keep us locked in the back of the minicab, with a driver who barely speaks English, going in a direction we don't want to go".

Not my brand of humour but then I don't have any neck tattoos.

Johnson has form with jolly race jokes.

This is the man who described Commonwealth citizens as "flag-waving piccaninnies".

The man who characterised Tony Blair's international development agenda thus: "No doubt the AK47s will fall silent, and the pangas will stop their hacking of human flesh, and the tribal warriors will all break out in watermelon smiles to see the big white chief touch down in his big white British taxpayer-funded bird."

Sturgeon versus Johnson is more than a technical exchange about Britain's role in Europe. It will be an unspoken clash of ideals, of two Britains: Progressive, devolved, outward-looking Britain and privileged, reactionary, insular Britain. True, there is much to quibble about Sturgeon's social democratic self-image but with contrasts this vivid, nuance will pale to grey. (It's also true that Sturgeon wants to break up Britain but she's evidently not that bothered at the minute.)

Sturgeon can point to -- frankly, she can just implicitly embody -- the difference in values between her Big Europeanism and Johnson's Little Englanderism. You're not just deciding Britain's relationship with Brussels, she can underscore for the voters, you are choosing what kind of country Britain is.

There are reasonable arguments on both sides of the referendum and on the Leave side a credible case against Brussels as an economic liberal bloc long since past its "social Europe" phase. And at risk of affronting the unquestioning Eurofederalism of civic Scotland, there are sound right-wing rationales for quitting Brussels too. There is too much regulation, not enough accountability, and sufficient bureaucracy to revive Yes, Minister and keep it fresh for a good ten, twenty seasons.

Vote Leave Scotland has been an altogether more nuanced, more decent affair than its noisy sibling south of the border. (Scotland Stronger in Europe too has raised the tone of the debate where the Britain-wide In campaign has played voters for fools.)

Years spent maligning Brussels for his backbench audience meant David Cameron could never offer a positive vision of Britain's role in Europe. Jeremy Corbyn's ambivalence about the European project has rendered his interventions bloodless and the chaos of his leadership was always going to distract from any part he could play.

Nicola Sturgeon has dedicated her adult life to splitting Scotland from the rest of the UK. Now it falls to her to make the case for unity and the pooling and sharing of resources. Get over the irony, hush the accusations of hypocrisy. The two campaigns are neck and neck in the polls. Tonight, Sturgeon could give Remain the final nudge it needs over the finish line.

Comment by Stephen Daisley, STV's digital politics and comment editor. You can contact him at