Who will blink first: Salmond or Cameron?

There is the Government's great White Paper hope, of course, the detailed blueprint due this autumn for how an independent Scotland would work in practice. But there is another possible event which those at the heart of the SNP are increasingly confident will turn the debate in their favour.

The post-budget chat in the parliament bar last Wednesday night did not, thankfully, dwell on John Swinney's hulking 194-page spending plan.

Instead, spin doctors talked about Alex Salmond's challenge for a head-to-head TV debate with David Cameron. The story is a well-worn one and looked for all the world to have ended in stalemate.

To recap, the First Minister says he must debate with the Prime Minister, at least in his first telly encounter. The Prime Minister says Mr Salmond should debate with Alistair Darling, as head of the Better Together campaign. So Mr Salmond accuses Mr Cameron of running scared, while Mr Darling says it's the First Minister who is ducking out.

The Nationalists are adamant the deadlock will be broken. They believe the Prime Minister's refusal to face the First Minister will become an increasingly painful running sore for the pro-UK side and ultimately prove "unsustainable", as they put it.

We'll have the debate, they say. See you in the green room. I'm not convinced they're right, though their absolute conviction is a sure sign we can expect a big Yes campaign effort to step up the pressure for a Salmond-versus-Cameron showdown sooner rather than later.

The arguments for and against the clash are couched in terms of high principle. The Nationalists insist that not only is the debate the one people want to watch, it's the debate they deserve. First Minister against Prime Minister is the appropriate match. The leader of Scotland, making the case for independence, and the leader of the UK, defending its continued existence. The pro-UK side says that while David Cameron has a view he doesn't have a vote. The debate should be a Scottish affair involving those seen as being at the heart of the rival campaigns: Mr Salmond and Mr Darling.

Those are the principles. You'll not be surprised to learn they float on a layer of purely political considerations. The most obvious is the likely outcome: it's pretty much impossible to imagine Mr Cameron coming out on top in the popularity stakes after 90 minutes in a television studio with Mr Salmond.

But there is also a strategic reason why the Prime Minister is resisting trial by television. The pro-UK camp is determined not to allow the Nationalists to frame the independence debate as Scotland versus the rest of the UK.

Indeed they want to characterise the coming year as a contest between Mr Salmond and the rest of Scotland. "It's Scotland versus Salmond," is a line Scottish Labour leader Johann Lamont has already started using at Holyrood.

The No side's fear of letting the independence debate begin to look like something that might be played at Hampden Park or Wembley is why I think we'll not see Mr Salmond and Mr Cameron go head to head.

There are only two scenarios I can envisage which might bring it about: if the Nationalists took a clear lead in the polls, when Mr Cameron may indeed come under irresistible pressure; or if the polls widened to the point it didn't really matter.

Even then he would be extremely wary of joining battle on his opponents' terms. The broadcasters hands are tied, by the way. They know a Salm-Cam show would be a ratings winner but equally they know they can only screen what the politicians would agree to do. While Have I Got News for You can field a tub of lard in the event of a no-show, we can be reasonably sure the referendum rules will be stricter.

This cuts no ice with the Scottish Government spinners. Chatting on Wednesday they refused to entertain the notion that Mr Cameron would carry on ignoring the challenge. But what if he does?

Mr Salmond, who cannot go the year without taking part in a single televised debate, would surely have to blink first.