The biggest surprise about this long General Election campaign is how little people seem to understand how the Westminster system works.

Even two days before the closest election in modern history, where neither of the big parties has a remote prospect of an outright majority, politicians are still saying that the largest party gets to be the government.

Not in our parliamentary system it doesn't. As Lord O'Donnell, the former Cabinet Secretary has explained, the largest party does not become the government unless it commands a majority of votes in the House of Commons.

Yet "senior Labour figures" were reported in The Times yesterday as saying that, if their leader Ed Miliband does not lead the largest party after Thursday, then his government would not be "legitimate".

One Labour front bencher is quoted as saying: "If we come second and try and cling on, [everyone else] will kill us."

This is not only defeatist (don't they want a Labour government?); it also fails to understand how a government is formed and sustained in a parliamentary democracy.

The constitution imposes obligations on those who participate in a parliament. The first responsibility of all party leaders in Westminster is to ensure that the UK has a stable government.

It is actually quite difficult for a party leader who is able to form a governing majority not to do so.

Let me explain. Imagine if Mr Miliband were to say on Friday morning: "OK, the game's up. I may have the numbers to form a government with other willing parties, but I feel it would be illegitimate and the newspapers will hound me. I would rather the Conservatives continued in government than enter Number 10 on the strength of nasty nationalist votes."

So what happens then? David Cameron and the Liberal Democrats will dither for a while, arguing about whether to have a referendum on Europe. Then Mr Cameron will put his Queen's Speech to the House of Commons at the end of May.

But Labour and the SNP, if the polling figures are right, will vote down that Queens Speech, along with any confidence motion attached to it. This is not a nationalist take-over; it is simple arithmetic. On the current polls, no Tory confidence motion could possibly pass.

Take, for example, the latest poll of polls in the New Statesman's May2015 blog: Tories 273; Labour 269; Liberal Democrats 27; SNP 56. Even with the nine DUP MPs on his side, Mr Cameron will not be able to outweigh those numbers.

The SNP cannot be expected to vote for a Tory government they have pledged to oppose, and neither surely can Labour. The only way Mr Miliband could not end up as prime minister in these circumstances would be if he urged Labour MPs to abstain in the vote on the Tory legislative programme.

Perhaps that is what those anonymous voices on the Labour front bench expect him to do. There are some serious advocates of a "grand coalition" between the Tories and Labour. Some on the Blairite wing of the Labour Party never wanted Mr Miliband to become leader, let alone prime minister.

But it would assuredly be the end of the Labour Party as we understand it if Mr Miliband were presented with the opportunity of forming a government and then turned it down.

He would be accused by Labour members of surrendering the poor and low paid to the predations of Tory hair-shirt austerity, standing idly by while Mr Cameron and friends privatise the NHS, impose £30 billion in cuts and take Britain out of Europe. Most Labour voters aren't so picky about whom they deal with in a parliament of minorities.

This does not mean that Mr Miliband should pile in on Friday and try to form a government right away. The sitting prime minister has the right in our system to try to form a government. But it is always down to MPs in the Commons to decide whether or not it should continue.

Were this principle to be changed and, as some are suggesting, the largest party were automatically to become the government, then that would be an entirely constitutional arrangement.

Parliamentary sovereignty would be at an end and Britain would have introduced what is effectively a presidential system of government. Call me old fashioned, but I just don't think that is going to happen.