There is rarely anything new under the sun, particularly when it comes to politics.

Delve into the digital world of YouTube and you'll find a 1995 encounter between Alex Salmond and George "devolution will kill independence stone dead" Robertson, going head to head at the Old Royal High School.

Both the topics (currency, the EU, oil and devo pledges) and faces (Messrs Naughtie, Marr and Riddoch) are familiar, but my favourite bit is when the SNP leader - on fine form, incidentally - cites the difficulty of using Scottish banknotes in London and abroad.

"If Scotland has an independent currency," he declares at one point, "at least we'll have a first-class currency like everyone else and not a second-class sterling as we have now." His supporters (which include even more familiar faces) cheer, and Mr Salmond looks rather pleased with himself.

Plus ça change, only nearly two decades later the First Minister looks less comfortable discussing currency; indeed, one of the key moments in STV's first TV debate came when, under sustained fire from Alistair Darling, Salmond visibly floundered over Plan B. Not, it might be said, a good look in front of 1.7 million viewers.

Tonight his line on sterling will be a little stronger and certainly more polished. It has to be, for Darling is sure to take a recidivist approach, highlighting (or so he hopes) that between the first and second debate his opponent still doesn't have any answers. Perhaps he ought to remind Mr Salmond of his confident prediction back in 1995?

According to reports, the former Chancellor will also focus on Sir Ian Wood's recent claim that the Scottish Government is seriously overstating future North Sea oil revenue, although if last week's First Ministers Questions is any guide then he won't get very far: Mr Salmond knows his way around the politics (and economics) of oil and landing a blow won't be easy. There are ample statistics, meanwhile, to support either analysis.

There's also the NHS, which the First Minister last week said had emerged "as the single key issue" of the campaign in terms of converting people to Yes, although of course that's only because the Yes campaign has tried very hard to make it a "key issue", on largely spurious grounds.

As another former Chancellor, Lord Lawson, observed in his memoirs, the National Health Service is "the closest thing the English have to a religion", although the same is true in every part of the UK, not least Scotland. With polls having indicated that a majority of voters believe the NHS would get worse under independence rather than better, Yes strategists have gamely tried to turn a negative into a positive.

And polling suggests they're having some modest success, although that doesn't mean it's a good argument. Rather, as David Cameron bluntly put it last week, it is political "desperation". Raising the spectre of privatisation taps into fears about the loss of an institution that is cherished in Scotland while playing on the fact a lot of Scots aren't aware that health has been fully devolved for the past 15 years. It is, therefore, cynical as well as desperate.

Interestingly, the Yes campaign has had its NHS line in reserve for some time, having first floated it back in April when the Scottish and UK cabinets ended up within miles of each other in Aberdeenshire. Only in the last few weeks, with No still ahead in the polls, has it been ratcheted up.

But it almost goes without saying that a televised debate is not the best context in which to explore such complex issues. Much of what takes places at Kelvingrove this evening will soar over a lot of viewers' heads: the usual lies and damned lies, claims and counterclaims. Of course the commentariat will have lots to write about, but undecided voters - as on August 5 - will be left none the wiser.

For that reason and others, part of me remains uncomfortable with the televised political debate, for it seeks to graft a legitimate feature of US politics on to the UK's Parliamentary system. In 2010, for example, viewers were invited to believe they would somehow be voting for Gordon Brown, David Cameron or Nick Clegg when of course only relatively few would be doing so.

And while there is an argument that the medium is better suited to a referendum, particularly one presenting voters with a binary choice, it still ends up over-personalising the process. For all the Yes campaign's cries of "it's not about Alex Salmond", that's certainly how it ends up appearing when the debate is reduced to a televised joust between two politicians in suits.

Still, such are the demands of the modern media age, and there certainly appears to be an appetite (however unsatisfied it might turn out to be) among Scots. A lot of those watching will be voting by postal ballot and, given that these will be distributed in the days following tonight's debate, it's importance cannot be underestimated.

What's unlikely to manifest itself on the box is a noticeable increase in paranoia and silliness as the long campaign draws to a close. I now can't attend an event without someone mentioning secret oil riches (Clare and Clyde) hidden by the duplicitous "Westminster system", the prospect of English "punishment" following a No vote (including the abolition of Holyrood), and many other referendum tales tall and (un)true.

Yesterday I read of Yes supporters planning to use pens to mark their ballot papers amid fears their pencilled crosses might be altered, and, while that might be an extreme example, it's an inevitable by-product of the Yes campaign's negative phase, not just scaremongering over the NHS, potential cuts to the Barnett Formula (what do they think would happen to it after a Yes vote?) and the prospect of government by Boris/Farage/insert political bogeyman here.

My favourite was the line (distributed by the Yes campaign) from the so-called "godfather of devolution" Canon Kenyon Wright: 'No is a yes to Margaret Thatcher and the damage she wrought on our country, a yes to Tony Blair taking us to war on a lie, a yes to George Osborne imposing the Bedroom Tax and yes to David Cameron and the Tories taking us out of the EU."

Given that Alex Salmond once claimed the pro-independence campaign was "positive, uplifting, hopeful and must always stay that way", there has been an obvious shift in tone, and one that seems set to continue over the next few weeks. All remaining nuance or ambiguity will disappear and instead we'll be repeatedly thumped over the head with an increasingly stark choice: good Scotland versus bad UK.

As the Yes Scotland strategist Stephen Noon has outlined, it's the final of three campaign phases: we can do this; we should do this; we must do this. The last three weeks of the referendum will be framed as a vote for or against Scotland. A No vote might even be presented as a rejection of the very concept of Scottishness. Watch this space.