IT was one of Bill Clinton’s former campaign chiefs, James Carville, who coined the phrase that is believed to sum up the art of winning elections: “It's the economy, stupid.”

But if Carville’s celebrated slogan was relevant to Scottish politics, opponents of independence would have their feet up and be relaxed about the future of the Union.

In 2014, Better Together ran a brutally effective campaign that tore into the economic basis of Alex Salmond’s independence plan.

Two years later, the SNP Government’s independence White Paper has aged badly: the oil price has collapsed; Scotland’s deficit has ballooned; and the idea of a ‘currency union’ with sterling is under review.

And yet, despite the battering that independence has taken, the SNP’s mission of taking Scotland out of the UK remains firmly on the agenda.

First Minister Nicola Sturgeon has proclaimed that a second referendum, post-Brexit, is “highly likely”; legislation on a new plebiscite is in the pipeline; and opinions polls, while not showing Yes in the lead, show support up to around 47%.

Interviews with senior figures who backed No in 2014 reveal a mixture of confidence, frustration and nervousness about a second referendum.

The confidence comes on the financial front. If another vote was fought purely on economics, the fight would be stopped in the early seconds of the first round. The White Paper, as even senior SNP supporters would concede, is dead in the water.

Ruth Davidson, the Scottish Tory leader, says that, in light of recent economic events, the assumptions of the White Paper were “unforgivable”.

She explains: “They were trying to suggest you could have this huge constitutional change with absolutely zero economic impact. It was simply a lie.”

Tom Harris, a former Labour MP who headed the pro-Brexit campaign in Scotland, concurs: “Logically, if you look at the economic argument, independence shouldn’t even be on the agenda, if you are looking at the GERS Government Expenditure and Revenue Scotland) figures, the oil price figures and deficit.”

The prospect of a refreshed economic strategy for independence – with Sturgeon admitting risk – does not worry No supporters.

Willie Rennie, the leader of the Scottish Liberal Democrats, says: “It would be more credible, [but] they would lose again. All we were asking them to do was just to admit reality. I don’t think it’s really about presentation.”

Davidson broadly agrees: “Had there been a more realistic case, it may have garnered more support, [but] that’s not been tested or fire-tested in battle so it’s a hypothetical.”

However, therein lies the problem for supporters of the UK. Despite Carville’s wise words, the economy is one strand of the independence debate, not the full tapestry.

Currency, deficit and oil are important for voters, but so are identity, democracy and Europe.

For Harris, many voters consider independence from an “emotional point of view”, while Davidson says she was taken aback at some of the feelings unleashed in 2014: “The referendum debate went past economics. For me, I was surprised at how much personal identity I felt was wrapped up in this.”

Johann Lamont, who was Scottish Labour leader during the referendum campaign, says: “Some of it is nationalism. We know that sense of identity is stronger now across Europe than it was in the past. I think we are fortunate and blessed in the way in which it, in general terms, expresses itself in Scotland.”

On the same theme, Rennie is perplexed at the way voters can park concerns about the economics of independence:“I am surprised by some people who, when I say to them ‘does the plummeting oil price have an effect on whether you vote Yes or not?’, they say ‘no, the oil will come back’. It is a casual dismissal which still surprises me now.”

Although No won relatively comfortably in 2014 – two million people endorsed Scotland’s place in the UK, against 1.6 million for independence – the numbers are tight.

If - and this is subject to caveats - Yes held on to every vote won two years, victory could be secured by peeling off one in ten No voters.

Harris, despite being a leading Brexiteer, believes the politics of leaving the EU is the biggest short-term threat to the UK: “It is a threat because it gives them a political opportunity.”

He also warns: “These are worrying times for those of us who still support the UK.”

However, Harris ultimately believes the shot in the arm given to the independence movement will wear off:

“The emotional resonance people have with the UK, whether you are for or against it, is far, far, far greater than any affection or ambivalence towards the EU, which is why there hasn’t been that shift as a result of the Brexit vote.”

Rennie believes “some” liberal-minded Scots switched to backing independence immediately after Brexit, but insists the number has “plummeted”.

A bigger worry is the state of the Labour party and the effect it may have on a future vote.

Centre-left voters had various reasons for backing the Union, but two jump out: the belief that Scotland benefits from redistribution through the Barnett Formula; and the hope that a progressive Labour Government lay round the corner.

The Barnett Formula guarantees Scotland’s special financial relationship, but the prospect of a Labour government in the next decade – due to civil war over Jeremy Corbyn – is almost non-existent.

Harris says: “There are certain advantages she [Sturgeon] has this time that Salmond didn’t have in 2014. One is no prospect at all of the Labour party ever forming a Government at a UK level again. That is very significant.”

Lamont does not say Labour’s woes are a threat, but notes: “We need to look as if we can win at a UK level.”

Rennie does not believe Labour’s implosion will affect the dynamics of independence, but says it is unhelpful: “The Labour party is not the only vehicle for progressive change, but it is a problem, at least in the short-term.”

He believes the bigger threat to the Union is the behaviour of the Tory Government.

“The Conservatives bungled the post-referendum victory by turning it into English nationalism and he [David Cameron] followed that through into the general election. He cocked up Brexit, which has put more strains on the Union,” he said.

Simon Pia, who advised two former Scottish Labour leaders, was a soft No voter who has since changed his mind.

Recalling September 18 two years ago, he says: “With a heavy heart, I voted No.”

The EU referendum and Labour’s woes have forced a rethink: “Brexit has such long-term ramifications for my children, grandchildren...it shows that the constitution of the UK is so flawed.”

He also says there are more Labour voters like him: “I was quite struck, in the days after Brexit, how many of them said they would be prepared to would vote Yes. It wasn’t the daft emotionalism of the Nationalists, but it was fairly realistic and calm.”

He sums up his change of position: “It’s the politics of it, not the economics.”

Jamie Glackin, a former chair of Scottish Labour who still sits on his party’s Executive, is also having second thoughts about independence: “The impact of Brexit could be absolutely catastrophic on the energy sector [where he works], particularly renewables. I am far more likely to vote Yes than I was.”

His party’s turmoil at Westminster is another factor: “If Jeremy Corbyn is returned Labour leader, I find the prospect of a Labour Government incredibly unlikely.”

Agreeing the structure of the pro-UK campaign group in the event of a second referendum will produce another headache.

Better Together was poison for Labour at the ballot box and there is little appetite within the party for another cross-party alliance with the Conservatives.

Harris believes Labour would stay out this time: “The Labour Party will refuse to get into another Better Together-type coalition - they will be wrong in doing so, but it is not the first wrong decision the Labour party has made since 2014.”

For him, Davidson would inevitably lead the group: “Ruth is now the leader of the Opposition in Scotland. She would have to lead that campaign and that would be difficult for the Labour party.”

However, Rennie warns against a Davidson-led organisation: “It’s really important there’s a progressive, centre-left moderate, pro-UK voice that is very strong, distinct and clear in this campaign. That’s why we can’t leave it to Ruth Davidson.”

Many No figures are confident of winning a second referendum, but are anxious about the wildcard elements evident in global politics: Brexit; Syriza; the rise of Donald Trump; and the general feeling that voters are in the mood to give the status quo a bloody nose.

As Carville might have said: “Not just the economy, stupid.”