It’s possible that the Tories may yet pull it off, as their Australian alchemist, Lynton Crosby, has always promised. The main parties may still be almost neck and neck. But undecided voters could break to the Conservatives. Soft Ukip supporters could finally deliver David Cameron the votes he needs to stay in Downing Street.

If so, we know what to expect. With or without Nick Clegg, it will mean even deeper austerity, harsher cuts to social security, accelerating NHS privatisation, more attacks on workers’ rights, new handouts to the wealthy, more poverty and job insecurity, and perhaps another downturn in the slowest economic recovery on record.

But so far the numbers still aren’t there. So in case Thursday’s election doesn’t deliver a parliamentary majority for Cameron – even with the support of the Liberal Democrats, Ulster unionists and Ukip – the Tories and their media cheerleaders are moving to implement Plan B. After weeks of stoking English nationalism and painting the Scottish National Party as a mortal threat, aimed at sapping Labour support north and south of the border, the Tory machine has a new focus: any government led by Ed Miliband and dependent on SNP votes, Conservative politicians and their press pack now claim, would be “illegitimate”.

The home secretary, Theresa May, declared that it would create the “worst crisis since the abdication”. Now the prime minister and his allies insist a Labour government would be a “con trick” – and, in an ominous vein, that Miliband is out to “seize power” without winning the largest number of seats.

The press onslaught on Labour now outstrips even that meted out to Neil Kinnock in the 80s and 90s. The non-dom- or tax exile-owned Mail, Express, Telegraph and Murdoch media groups have unleashed an avalanche of propaganda against Miliband, whose modest break with the political and corporate consensus has created something close to panic in parts of the establishment. His plans to tax the rich, and non-doms in particular, make it personal.

So Conservative politicians and their proprietor friends are determined to make sure that if anyone is going to seize power, it’s them. If Miliband fails to win the largest number of seats, Rupert Murdoch’s Times declared on Monday, Cameron should “occupy Downing Street”, regardless of whether he has a majority in parliament.

Cameron’s plan, he has let it be known, is to declare victory, cut another deal with Nick Clegg, and sit tight, backed by a media barrage and regardless of whether their parties are outnumbered by an anti-Conservative bloc in the Commons.

But to claim that only a party that wins the largest number of seats can form the government is to turn the parliamentary system on its head. As numerous precedents, the government’s own “Cabinet Manual” rules and common sense dictate, it’s only possible for a party leader who can command a majority in parliament to put together a government.

If, as the polls suggest, the Tories end up marginally ahead on seats and votes, Cameron can try to form an administration with the support of the defeated Liberal Democrats, the regularly homophobic Democratic Unionist party and the racially inflammatory Ukip. But if they can’t reach the effective majority of 323, then it falls to Miliband to assemble a government that can.

If it won the support of a majority of MPs, to reject that government as illegitimate and seek to bring it down would effectively be to support a constitutional coup. But that is exactly what the Conservatives and their friends have made clear they intend to do.

The idea is for Cameron to cling on to power and try to break the resolve of a Labour-led parliamentary majority to vote him down. Even the oligarch-owned Independent, now backing the Tories, claimed yesterday that Britain faced a democratic “legitimacy crisis”.

It doesn’t. If Miliband were to command a majority in parliament, it would be likely to include Labour, the SNP, the Greens, Plaid Cymru and the SDLP, among others, representing perhaps 44% of the electorate. A Labour-Lib Dem deal would take that well over 50%.

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On the other hand, if Cameron were to agree a new coalition with Clegg, it would be on the basis of maybe 42% of the vote, down from 59% when the first Tory-Lib Dem deal was struck in 2010. He might be able to tack on DUP and Ukip support, but teaming up a party that has shed numerous seats with the biggest loser of the night scarcely sounds like a model of democratic legitimacy.

If Cameron fails to form a government, the political and media establishment will pull out every stop to prevent Miliband becoming prime minister. It won’t just be a wall of noise about “legitimacy” and chaos. Already some rightwing Labour figures are being primed to try a mini-coup of their own, echoing the Tory claim that the second largest party shouldn’t lead a government. If Cameron and Clegg come close to a majority, a handful of Labour defections could even take them over the line.

Miliband hasn’t made it any easier for himself by giving credence to the insidious claim that any deals with the SNP – likely to be the third largest party – would be beyond the political pale. Some in the Labour leadership would welcome a pact with the Lib Dems, not only for stabilising ballast in parliament but as insulation against the potential influence of the Labour left. But that might require the head of Clegg. If all else fails, the Tories and their media friends will try to force another election.

The only way to counter the onslaught that will be unleashed if voters return an anti-Tory majority is to turn the tables on the real losers. It would be a naked struggle for power in which the other side certainly won’t play by the Queensberry rules. But if Cameron and Clegg lose control of parliament in this election, it will be a rejection of the Westminster establishment and a mandate for change. For the sake of the disabled, food bank users, zero-hours contract workers and those crushed by the bedroom tax, among millions of others, that would need to be turned into a change of government and political direction. Far from being illegitimate, it would be the democratic outcome.