The election is on a knife-edge. But let’s imagine this: the Conservatives win the General Election on the morning of May 8th, but Cameron is just weeks, if not days away from resignation.

The Prime Minister has stated his intention to serve a full second term if returned to office, so why is his resignation the most likely outcome, even if he wins in two weeks time?

Principally, because the small margin by which the Conservatives will win the election, if indeed they do win, means that another Hung Parliament will be produced. This is crippling to the Conservative’s ability to govern – not because they won’t have the largest number of MPs – but because the Tories will unlikely be able to secure enough minor parties to join a formal Coalition so as to keep the government stable. This is because their Coalition members of the last five years, the Lib Dems, are set to return around half the number of MPs they currently have, and the combination of any other centrist or centre-right parties which may want to enter the fray just don’t make up the shortfall.

Perhaps stability is overrated. Perhaps the Conservatives could rule, for a time, as a Minority Government. They’d negotiate on a vote-by-vote basis using issues that are watered down enough so that even rivals in the House may be able to get on board and pass the odd bit of legislation. Not particularly satisfactory for anyone, but at least Cameron would be PM and the Conservatives would set the agenda. But no. One unprecedented factor hangs over any prospective government which may form after May 8th, and it’s the fact that an explicitly anti-Tory party is set to replace the more moderate Lib Dems as the third largest in Westminster – the SNP.

This means that if the Conservatives don’t win an overall majority, any attempt at Minority Government on their part will fail. Here’s why:

The SNP, for better or worse, has only one place to invest its potential voting power in the new Parliament. This follows Nicola Sturgeon’s declaration that she would join forces with Ed Miliband in the event of a Conservative Minority Government to vote down the Queen’s Speech. If more MPs vote against a Queen’s Speech than for it, the government collapses, and as Labour will by default vote against the Tories, which way the SNP sides would ultimately determine how this critical event pans out. The fact that they’ve already put their eggs in the Labour basket is thus hugely significant.

Looking back at the last time the SNP held significant sway in the Commons, it’s understandable why Sturgeon has taken this stance.

In 1979, the prospect of a referendum for a new Scottish Parliament had put the SNP in the national spotlight, and had as a consequence allowed them to secure more MPs at Westminster than they have ever managed since. However, the devolution referendum of the 70s would ultimately fail, largely because the SNP were seen to collaborate with the Conservatives in voting down the then Minority Labour Government. The election that followed swept Margaret Thatcher to power, a result that left the SNP bitterly portrayed in Scotland as backing the wrong horse. Sturgeon will know well the historic mistakes of her party, which delayed devolution by two decades and threw her predecessors into years of crisis.

The SNP are set to wildly outperform their 1979 counterparts this time around, however, and return more MPs to the Common’s than all of the other minor parties combined – the Lib Dems and UKIP included. Short of a status-quo shattering earthquake, all opinions polls are consistently showing exactly this result.

This is why it is arguably irrelevant if the Tories win the popular vote and even get the largest number of MPs in two weeks time. They need to get at least half of all MPs to secure their ability to pass the Queen’s Speech, because the huge cocktail of minority parties which will populate the rest of the Commons will not be sufficient to out-vote the SNP.

So then, David Cameron’s fate aside, it’s worth considering the constitutional crisis which may arise if the SNP manage to install a Minority Labour Government over a Conservative one which had received more votes. Not only will the SNP be able to dictate certain terms to Labour which will likely benefit only Scotland, but Labour will have to defend every move it makes as being independent from Sturgeon’s anti-UK agenda. It is for this reason that maybe, just maybe, Labour would maintain a Minority Conservative Government and pass the first Queen’s Speech because taking up the SNP’s offer would simply be too costly for them.

With an Overall Majority for the Tories becoming ever less likely as election day approaches, Cameron’s fate will be decided by the “will he, won’t he” of Miliband’s relationship with Sturgeon.