As most political wonks will know, the Conservative Party strategist Lynton Crosby hails from Australia.

 

A few years ago he was tasked with winning the 2015 general election for the Tories, and if the Labour frontbencher Sadiq Khan is to be believed, then the Wizard of Oz's fingerprints are all over David Cameron's most recent pre-election tactic.

Yesterday the Prime Minister called on Ed Miliband to rule out a post-election deal with the SNP. "If you thought the worst outcome in this election is a Labour government led by Ed Miliband, think again," warned Dave. "You could end up with a Labour government led by Ed Miliband, propped up by Alex Salmond and the Scottish National Party."

Then, resorting to what Dr Johnson called the last refuge of a scoundrel, Mr Cameron played the patriotic card, saying that Mr Miliband ought to follow his advice "if he cares about his country". Interviewed by the BBC on Sunday morning, Mr Khan said this continuing focus on an SNP/Labour coalition represented a "ju-jitsu" move from Mr Crosby.

He had a point, for the Tory tactic is obvious, although it's pitched at Middle England rather than persuadable voters north of the border. Already upset at the advantages (both perceived and real) enjoyed by Scotland within the Union, Mr Cameron is cleverly adding another constitutional bogeyman to the mix: the prospect of Labour "losing" a UK general election but nevertheless forming a government due to support from the SNP.

In Scotland, ironically, this is exactly the scenario being talked up by the still-surging SNP, and although there are obvious contradictions (Labour, according to the Nationalists, are both "red Tories" and a better bet than the Conservatives), it's clearly an attractive electoral pitch for left-leaning, Yes-inclined Scots.

Last week was full of what the media likes to call "senior" figures - including former Prime Minister Sir John Major - urging the Labour Party to cut the SNP adrift, although it has quite rightly done nothing of the sort. All the Shadow Energy Secretary Caroline Flint said yesterday was that her party did "not want", did "not need" and did not "plan to have any coalition with the SNP".

That, of course, was a straw man, for most of the speculation focuses on an informal Parliamentary arrangement rather than a 2010-style deal between a large party and a smaller one. First Minister Nicola Sturgeon has been quite clear that she instinctively favours, if anything, a "confidence and supply" arrangement with the Labour Party.

But even that has more to do with positioning than any serious intention of "propping up" (as David Cameron would call it) Ed Miliband. The urging, meanwhile, hasn't come only from Conservatives but from certain voices within the Labour Party, a classic case of bubble analysis being played out before a generally indifferent electorate.

It's wrong-headed on a number of levels. First, ruling anything out when several polls show Labour and the Conservatives at level pegging in terms of seats would be bad politics, while second, "ruling out" any arrangement with the SNP wouldn't make the blindest bit of difference: all the evidence suggests the Scottish voters Labour needs to lure back simply aren't listening.

All of the above, meanwhile, serves as a reminder of the UK's relative immaturity when it comes to coalition politics. In Lynton Crosby's native Australia, for example, the current Prime Minister Tony Abbott fought the September 2013 election as head of an opposition coalition comprising his own centre-right Liberal party and the National Party.

Similarly, in German federal elections it's generally well understood by the electorate whom is likely to coalesce with whom (i.e. the Greens with the centre-left SPD and the Free Democrats with the centre-right CDU); the surprise Tory-Liberal Democrat coalition formed after the May 2010 election means there are no such certainties in the UK.

Perhaps we'll get there over time, assuming, that is, there's still any need for UK general elections over the next couple of decades. There's also been some commentary suggesting that the SNP forming part of a UK administration (however informally) would somehow be politically anathema given its commitment to independence.

There is something in this, although again it's uncharted territory in the context of British politics (the old Irish Parliamentary Party was devolutionist rather than secessionist). In Canada a few years ago the then Liberal Party leader Michael Ignatieff was faced with a similar situation when, in the weeks before a federal election, the Conservative Prime Minister Stephen Harper repeatedly raised the prospect of the Liberals coalescing with the NDP and Bloc Québécois.

Mr Harper, like Mr Cameron, consistently framed his political opponents as part of a coalition, capitalising on the public reaction to an attempted partnership between the Liberals and NDP a few years earlier. Thus during the election campaign Ignatieff, NDP leader Jack Layton and Bloc leader Gilles Duceppe were all asked whether they'd try once again to form a coalition government, but Ignatieff had dodged the question.

So, eventually, Mr Ignatieff issued a statement. "We will not enter a coalition with other federalist parties," he told the media. "In our system, coalitions are a legitimate constitutional option. However, I believe that issue-by-issue collaboration with other parties is the best way for minority Parliaments to function."

He went on to "categorically rule out" a coalition or formal arrangement with the Bloc Québécois, a prospect polls had suggested many Canadians found distasteful given that party's commitment to independence for the French-speaking Province of Quebec. Mr Ignatieff's reasoning went beyond tactics: visiting Edinburgh a few months ago he told me that on philosophical grounds he just couldn't countenance the idea of a party committed to breaking up the federation forming part of its government.

Many Labour MPs in the UK will doubtless feel the same way (in Canada it proved an academic discussion, for on polling day both the Liberals and Bloc Québécois were trounced), and indeed an important factor that seems to have been ignored in most pre-election commentary is the sheer tribal hatred between Labour and the SNP. And if anything it's likely to get worse.

Picture the (increasingly likely) scenario: on the morning of 8 May dozens of Labour MPs will face the media having lost their seats to Nationalists in the unlikeliest of constituencies, including several senior figures. Even if the numbers add up, it's almost impossible to believe that Ed Miliband will then swallow his pride, ignore the advice of (former) Parliamentary colleagues and invite Nicola Sturgeon for talks.

Even if that happened, the SNP would arrive with so many "red lines" that even an informal deal would mean the Labour leader having to agree to three impossible things before breakfast. Perhaps realpolitik would trump all of that; perhaps the necessity of forming an administration lest the markets take fright would make friends of foes, but somehow I doubt it.