THERE'S a Japanese proverb loved by politicians: "If you sit by the river long enough, you will see the body of your enemy float by." Cannier souls at Westminster and Holyrood know that sometimes all you need to do to win is lean back and let your opponents self-destruct.

So far, it has proved a remarkably efficient strategy for Alex Salmond and his plan for a referendum on Scottish independence.

Last week, after a ham-fisted intervention by the LibDem Scottish Secretary Michael Moore, who suggested two referendums were needed before separation, David Cameron moved to take control of the issue, setting up a Cabinet sub-committee dedicated to maintaining the union.

He and his wife Samantha will visit Scotland next week to drive the message home.

But instead of quaking in their boots, the SNP are privately delighted, the Sunday Herald understands.

The Nationalists reckon that Cameron becoming the default leader of the No campaign is a gift. Not only is his party still regarded as toxic in Scotland, the Prime Minister may soon be leading one of the most unpopular governments of modern times, as the full, bloodthirsty extent of the Treasury’s spending cuts hits home.

However much Cameron might try to present himself as part of a society-spanning coalition on the union, he will also be portrayed by the SNP as the voice of London patronisingly telling Scots what’s best for them.

One senior SNP source said: “As the events of the last week – with the nonsense of the two referendums – indicate, the UK Government are making it up as they go along. They aren’t thinking things through.

“At the Holyrood election, the Tories got their lowest share of the vote in Scotland ever – even worse than the 1997 wipeout – so the idea of the Tory leader in London leading the No campaign is a pretty disastrous start.”

Salmond himself tries to rise above the fray, but his appreciation of the Cameron factor is clear.

Asked by the Sunday Herald, if Cameron, as the Tory head of an unpopular government, would be an asset to the independence campaign, he said: “I’m sure the Prime Minister will see himself as the leader of the pro-union campaign as the First Minister would see himself as the leader of the independence campaign. That seems pretty logical.

“I can’t help him being a Conservative and I can’t help him being unpopular. I have to look after my popularity.

“It’s obviously his prerogative. I’m not going to pick the leader of the No campaign.”

No, he won’t be picking their campaign leader – but he could hardly have found a better one.

It’s not just a political hunch that a backlash against the Tories might help the SNP case. An academic study into the 1997 referendum which led to the Scottish Parliament found the voting pattern closely mirrored local party allegiance.

That referendum had been devised by the Labour government, with the then Scottish Secretary Donald Dewar spearheading the Yes-Yes campaign for a Holyrood with tax-raising powers, with vigorous support from the SNP and LibDems.

The Tories heavily backed the No-No campaign.

The study found that in areas where support for Labour was high in the 1997 general election, support for devolution was also high, partly because it was the anti-Tory position. Support for devolution fell where the Tories had done well in the election.

“In short, Labour areas were more strongly in favour of a parliament than Conservative areas,” concluded the authors.

Fast forward to today, and, in Holyrood terms at least, there are no “Labour areas” left.

Instead, the SNP have extended their dominion from the Highlands and north east into Glasgow, Lanarkshire, Fife and the Lothians.

Next year’s local elections may well see the SNP become dominant at council level too.

So by the time of the independence referendum, say 2014, the Labour strongholds which coloured the 1997 vote could have morphed into SNP ones.

If the pattern of 1997 is repeated, areas good for the SNP electorally should also favour their referendum position, either for its own sake or to raise two fingers to Cameron and Nick Clegg.

It helps explain why Salmond is keen to defer the referendum to the second half of the parliament.

He’s not just assembling his own case, he’s also waiting for the Coalition’s unpopularity to peak, probably when the nation is at its angriest about the cuts, and before any economic recovery can generate the whiff of a feelgood factor.

The fact a voter backlash next time could be against two opponents rather than just one, thanks to the Coalition, only improves Salmond’s odds.

Labour will also be speaking up for the union, but that means aligning themselves with the Tories and LibDems, not a comfortable position.

However, what has become overwhelmingly clear since May 5 is how unprepared Westminster is for all this.

For years, the unionist parties parroted the mantra that if a majority of MSPs wanted an independence referendum it would happen, while never seriously imagining that it could.

The SNP majority has exposed a crippling lack of foresight on how to respond politically.

Constitutional academic Alan Trench, author of the Devolution Matters blog, said the Coalition had actually cut back its staff on Scottish devolution as it assumed the Calman Commission at Holyrood and the Scotland Bill currently passing through Westminster would be the last word.

“There’s been little thought or contingency planning under this government,” he said.

“There’s no capacity whatsoever at the centre. There’s simply a big gap in thinking and understanding these things.”

In contrast, Salmond and his ministers have been working towards this moment their whole careers.

As a result, the Tories and LibDems are trying to play catch-up, and doing it badly. Their frantic, muddled efforts also give the lie to another self-soothing unionist myth: that Scots would never vote for independence anyway.

True, opinion polls consistently show most Scots oppose independence, though one last week showed a narrowing gap between the Yes and No camps.

But polls are only a rough guide to the future, not a guarantee of what you’ll find there.

A rollercoaster referendum campaign with Salmond pitched against an unpopular Prime Minister could turn received wisdom on its head.

Trench says the onus will be on Cameron and Clegg to lay a firm counter-offer before the voters at the referendum, perhaps the full tax and spending powers known as fiscal autonomy or “devo max”.

“People are unlikely to vote No to independence if there’s not an offer of something that fairly clearly matches what they want. If there’s just a general promise the UK government will do something undefined in the future... it would be a great mistake. Something concrete needs to be on the table as an alternative if the unionist parties are to win.”

Given that devo max would still be regarded as a good result for the SNP, it’s hard to see how Salmond could entirely lose such a referendum.

The First Minister’s view from the riverbank must look pretty sweet just now.