Imagine this. It’s Sunday 18 June 1815 near a little-known place called Waterloo. Battle lines are drawn when word comes down the line that Napoleon Bonaparte is to be declared the winner without a shot being fired. He, after all, has about 73,000 soldiers at his disposal, making his army larger than the one commanded by Wellington, who can only boast 68,000 (a sizeable proportion of whom aren’t even British). Never mind that Blücher’s Prussian force, supporting Wellington, is some 50,000 strong. The French, as the largest single contingent, have every right to run the show.

Absurd, no? Yet, if we fast forward two hundred years, this is exactly the argument that’s being made to support the idea that, should the Tories emerge as the largest party in another hung parliament after 7 May, they are somehow entitled to govern the UK for another five years or, at the very least, to get first go at trying to form a coalition or a minority administration.

This might be the common wisdom. But it is nonsense, nonetheless. As Napoleon found to his cost, you may have more troops than your nemesis; but if he’s managed to put together an alliance that outnumbers you then you will end up the loser, even if, to quote Wellington, it turns out to be a damned nice thing – “the nearest run thing you ever saw in your life”.

Numbers count. Of course they do. But when it comes to forming governments, physics always trumps maths. Who gets to govern depends on what force they exert within the system, singly and collectively. Clearly, having large numbers of MPs helps. But so too does being (to use the political science jargon) pivotal – capable of deciding, by virtue of your ability as a party to jump either way, which one of a number of potential combinations can actually govern.

That is why, even though they underplayed their hand woefully in the ensuing negotiations, the Lib Dems were actually in a very powerful position in 2010, especially once they’d decided they wanted to be in government rather than merely support one from the outside.

That is also why the Lib Dems (along with, if the polls are correct, the SNP and also the DUP) may well be in an equally (if not more) powerful position in 2015 – even if, this time around, they end up with only half the seats they won then.

But, some purists will cry, what about the constitution? Wasn’t Stanley Baldwin, with 258 MPs, allowed to “face the House” in 1924 before giving way (after losing a confidence vote) to a minority Labour government, led by Ramsay MacDonald, with just 191 MPs?

Well yes, but what of it? For one thing, Baldwin was playing a highly sophisticated game, having already made up his mind that it would be in the long-term interests of the country, as well as the Conservative party, to allow “the Socialists” their first crack at government under controlled conditions. For another, it was all a very long time ago – a precedent, maybe, but hardly a convention that can bind 21st-century politicians.

To think that those same politicians will be bound by the civil service’s cabinet manual is equally absurd. The manual is not holy writ. It can suggest but it cannot prescribe. Ultimately (to use the pol-sci jargon just one more time) it’s a case of freestyle bargaining. Coming out of the election with more MPs than any other party doesn’t grant you any special rights, even if you’re the sitting prime minister. Those outraged by all this may come up with one last argument, namely that the UK’s proverbial sense of fair play will be offended if the party with the most seats doesn’t get to govern, or at least get first go.

But why, either inherently or rhetorically, would that be any less fair than handing power to a party with what would still be less than half of the seats in the House of Commons and only just over a third of the votes in the country?

Right now, those most determined to create a self-fulfilling prophecy (or at least a degree of momentum) in favour of the largest single party are Conservatives – presumably because they believe they will be that party. It is perfectly possible, however, that Labour will finish just ahead of the Tories – and perfectly possible, as the result of choices made by the “minor” parties, that Ed Miliband might nevertheless be unable to wrest the keys to No 10 from David Cameron’s patrician grasp.

The UK’s first-past-the-post electoral system, it seems, is no longer able to prevent either its party system or its parliament from looking more European. So we need to realise, as they do on the continent, that sometimes those who “win” elections can still end up on the losing side.