The principal protagonists try to persuade us - and, even more, to convince themselves — that there will be a late switch that will send a decisive number of voters surging into either the red column or the blue one. There are many absurd aspects to this campaign, but the most pantomime feature of it is that both David Cameron and Ed Miliband have to pretend that they are going to be in charge of a single-party majority government after the election. The message of the polls, and it is a very consistent one, tells us the truth that they dare not address. Voters seem determined to deny the privilege of one-party rule to either of them.

True, there are still a lot of people saying they haven’t made up their minds yet. We should allow for the possibility, raised by Ed Miliband in the interview in today’s Observer, that enough voters might make a leap in one direction to give one of the bigger parties a parliamentary majority. But none of the forecasting models is projecting that outcome.

Even if one of the larger parties does upset expectations and manages to scrape over the line, we will still be ruled by a form of coalition. For the Labour and Tory parties are themselves agglomerations of ideologies.

The Conservative ranks include the veteran europhile Ken Clarke at the moderate end of the spectrum of the right and, at the other end, Tories with views that are indistinguishable from those of Nigel Farage. They are keeping quiet for now, but they haven’t gone away.

The many Tories who can’t stand David Cameron are still there, lurking in the bushes with their machetes. The Tory civil war about Europe is not over. They are merely observing an armistice that will end around 10pm on election night. Of the various potential outcomes of this election, a Cameron government trying to rule on its own and held to ransom by his wild things could be the most unstable example of — what’s the phrase? — “a coalition of chaos”.

he Tory leader won’t acknowledge that - even if Mr Cameron secretly fears it. Nor will the Labour leader address what might face him. “I’m not going there,” said Mr Miliband when we talked to him. He boringly rebuffed any questions about what he will do if he comes up short of a majority by saying that he would not talk about “hypothetical” scenarios. The Tory leader likewise refuses to be drawn into any conversation about what he will do in a hung parliament.

While point-blank refusing to say anything about their own plans, the Tories are entirely eager to speculate about what Labour might do. Vote Miliband, they say, and you’ll end up being governed by Alex Salmond and Nicola Sturgeon. That line of attack has been sharpened by the latest polling from Scotland. It is dire for Labour. If it translates into real votes in ballot boxes, the Labour casualties will be massive and include some of the party’s biggest names, including Jim Murphy, the Labour leader in Scotland, and Douglas Alexander, the party’s campaign chief. Who is cheering that prospect most loudly?

Well, it is obviously very pleasing for the Nationalists who currently occupy an extremely sweet political spot. It won’t last forever, but at the moment it is working hugely to their advantage that they are both governors in Edinburgh and insurgents against Westminster.

Nicola Sturgeon has greatly profited from the TV debates, which is quite a feat when she is not even standing as an MP. Her offers to support a Labour government are made in the same spirit that a rope supports a hanged man. Being the canny politician that she is, she must know that it feeds the Tory attack on Labour.

The success of the separatists has brought no less delight to David Cameron. His party may still trade until the title Conservative and Unionist, but its glee at the Nationalist surge becomes daily less disguised. If the Scottish National Party (SNP) is such a mortal menace that it would be unconscionable for Labour even to talk to the Nationalists in the event of a hung parliament, then it is fair to ask why Tory spin doctors have spent so much energy cheerleading the performances of Sturgeon in the TV debates.

They sometimes sound as if they are in love with her. It is because she is doing to Labour what the Conservatives are not managing to do themselves. It is the rampant SNP, not the Tories, who have so far done most to diminish Labour’s chances of winning a parliamentary majority.

The Conservatives are obviously trying to use the prospect of a big block of Nationalist MPs in the next parliament to scare English voters with the thought that a Labour government would be a marionette jerked around by SNP demands. This is about the battle for swing voters in Tory-Labour marginals, but it is not only about that.

It is also a less remarked upon dimension of the close struggle in seats where the principal combatants are the current coalition partners. In Lib Dem/Tory marginals, the Conservatives are sending out leaflets telling voters they have to elect the Tory to ensure that Mr Cameron has a majority to shut out the Scots.

Tory strategists claim that it is working because they hear it “coming up on the doorstep”. There’s some confirmation of that from Lib Dems who express alarm about the impact of the Tory tactic. Says one senior Lib Dem: “It is doing us quite a lot of damage.”

The slightly less obvious thing that the Tories are trying to do is make the idea of any post-election arrangement involving the SNP so toxic in the minds of English voters that it would seem illegitimate. Labour is visibly frustrated by how much air this is getting in the media.

Miliband has tried to shut down the Tory attack by saying many times and in explicit terms that he won’t go into coalition with the Nationalists, the position he repeated in his theatrical exchanges with the SNP leader at the climax of last week’s TV clash between them.

So the Tories have now changed the question by demanding that he rule out any arrangement of any kind with the Nationalists. Privately, Labour strategists argue that the SNP’s bargaining hand in a hung parliament would really be quite weak because they are sworn to do all in their power to exclude the Tories from office.

In our interview with him, the Labour leader suggests that he would try to call the Nationalists’ bluff. He would present his programme — and no doubt include in it things Scottish voters would find attractive — and dare the SNP to vote with the Tories.

That leaves a lot of unanswered questions about what Labour would do in a hung parliament, but it is still more answers than we have ever got from Cameron. If he needed the help of other parties to sustain a government, where would he look? Would he sup with the fruitcakes, as he once called Ukip, if a deal with Nigel Farage was the price of extending his lease on Number 10? What would the Tory leader bung to the Democratic Unionists to buy the support of the Ulstermen?

These are good questions and the Lib Dems have been trying to raise them by suggesting that Cameron could be held hostage in Downing Street by a combination of rightwing forces that they have dubbed “Blukip”. This is an attempt to flip the Tory argument about hung parliament “chaos” back against the Conservatives. It is also a way of trying to get the Lib Dems back into the national conversation. They are the one party explicitly campaigning for another coalition and offer themselves to centrist voters as the smaller party with a track record of proving that they can sustain a stable government.

One of the truths that dare not speak its name during this campaign is that another deal with the Lib Dems would be much more desirable to Cameron than having to rely on votes cobbled together from the DUP, Ukip and the Faragiste fellow-travellers on his own backbenches.

For Miliband, the Lib Dems would be much more congenial dancing partners than the Scottish Nationalists.

Not that either the Tory leader or his Labour rival will admit that before May 8. Both will continue to insist that Britain must vote for single-party government even if Britons are showing a strong disinclination to do that. Both will scorn the idea of parties working together after the next election. Which is going to make life that much harder for whoever is in Number 10 if they find that working with others is precisely what they do have to do.

Guardian News & Media Ltd