David Torrance: Why there is nothing new under the sun, especially in politics

There are, however, exceptions. Recently the Prime Minister demonstrated that an Etonian education isn't what it once was when he warned that for the "first time in our history" a group of Nationalist MPs might be able to "alter the direction of our country", seemingly oblivious that the Irish Parliamentary Party had once done exactly that.

Others retreat into hyperbole, a well-known refuge during close election contests. Yesterday Theresa May told the Mail on Sunday that a Labour government dependent upon SNP votes would amount to the "biggest constitutional crisis since the abdication" of Edward VIII in 1936, even though that was a crisis of monarchy rather than parliamentary government.

The Home Secretary's curious analogy distracted from her more valid point - one made in this column a few weeks ago - that many English voters would question the "legitimacy" of such a government. Yet in several respects, we have been here before.

Many who lived through it, for example, continue to view the 1970s as a low, dishonest decade precisely because it produced the sort of instability and uncertainty now being experienced in the run up to this election. Indeed, it's almost too neat a fit: economic turbulence, hung Parliaments, minority governments, multi-party horse-trading, the Scottish Question and even a European referendum.

Not to forget an insurgent SNP. As the writer and journalist Neal Ascherson recently observed, until now SNP popularity had generally "alternated between dizzy surges and long slumps". The first such "surge" came in the wake of Winnie Ewing's Hamilton by-election victory in the late 1960s, when the party polled 38 per cent in local government elections and boasted, by some undoubtedly questionable estimates, 120,000 members.

Then, at the 1970 general election, the SNP slumped, although not for long. By 1973, amid cries of "It's Scotland's Oil" and reluctant attempts to devolve power, Margo MacDonald romped home in a by-election and, in the elections of February and October 1974, it gained seven and then eleven MPs, surging to 22 and then 30 per cent of the vote.

And although Harold Wilson secured a slim overall majority following the second of those elections, by 1976 his government, and his successor James Callaghan, had lost control of the Commons through defections and by-elections. Technically, the SNP's "football team" of 11 MPs held the balance of power throughout the UK.

That, and the SNP more generally, scared politicians used to a more certain age in which 97 per cent of voters supported either Labour or the Conservatives. In 1971 Douglas Hurd (whose uncle Robert, incidentally, had been the SNP's first press officer) depicted a near-future election in his terrific thriller "Scotch on the Rocks". In it, a fictitious Tory government is compelled to negotiate with a surging SNP in the context of a hung Parliament.

Hurd also hinted at tartan terrorism, something that moved the SNP of the day to complain when his book was dramatised by the BBC. But it was a caricature even then. Recently a friend sent me an old SNP publication called "The Scotland We Seek", published on the cusp of the party's second surge in 1973. It makes fascinating four decades later: cool on the Common Market and cautious about "excessive" exploitation of North Sea oil, it's also moderately quite leftish, big on trade unions, "works councils", a minimum wage and "ending state support for private schools". There is no mention of nuclear weapons, indicating that that has been a more recent Nationalist concern.

But this agenda was no more extreme than that of the contemporary Labour Party, which of course possessed its own tabloid bogeymen. In the 1970s the "most dangerous" person in Britain was Tony Benn rather than Nicola Sturgeon, and in 1977 Mr Callaghan sought to neutralise the Bennites while also shoring up his government's position by negotiating a "pact" with David Steel's Liberals.

Indeed, the Lib/Lab Pact, which lasted for around eighteen months, presents the most recent Westminster precedent for what might happen between Labour and the SNP after 7 May, although the lesson (for the Nationalists) ought to be to proceed with caution. While the Labour government got stability and made a start on controlling rampant inflation, the Liberals received a few trifling concessions and got pulverised in the polls.

By late 1978 the Pact was over and Callaghan had ducked an early election, but in the pages of the "Scots Independent" newspaper a young SNP activist called Alex Salmond, recently graduated from St Andrews, attempted to influence his party's strategy ahead of a Queen's Speech anticipated that November.

The young Alex ruled out "pacts or agreements" (that idea, he said "would be rejected out of hand by all responsible nationalists") but argued that the SNP's 11 MPs ought to seize the "political initiative" by "stating and restating what the SNP believe should be in the Government's programme". He mentioned the still-familiar examples of an oil fund, an early referendum date (for a Scottish Assembly rather than independence) and "safeguards against nuclear dumping".

Then, argued Salmond, if Labour's Queen's Speech "met a substantial part of the SNP programme" its MPs could sustain the government in office, and if it did not, they "could bring them down safe in the knowledge that they had clarified the issues on which the election in Scotland could be fought and won". The context was important: during that Parliament the SNP group had often appeared strategically chaotic, while there was ongoing tension between the party's left-leaning leadership in Edinburgh and its more conservative representatives at Westminster.

Although that tension no longer exists to any serious degree, it just might become an issue after this election. If, as looks likely, there is a band of between 40-50 SNP MPs in the House of Commons, that group will be almost as numerically strong as that at Holyrood while stewarded, in effect, by two different leaders: Ms Sturgeon in Edinburgh and her predecessor in London. Now it seems unlikely the First Minister will ever lose control of her party's southern contingent, but nevertheless Mr Salmond's penchant for freelancing might cause trouble.

Therefore, as the political scientist James Mitchell has argued, Westminster could end up having more of an impact on the SNP than it has on the House of Commons. As Salmond presciently observed back in 1978, urgent discussion was necessary "if present and future Westminster balance of power positions" were "to be made to work for us rather than allowed to work against us". That remains true today, confirming the old adage that there is nothing new under the sun, especially in politics.