What is it about Alex Salmond that makes otherwise sensible people turn into barking loons?

The normally equable columnist Alex Massie went into demented Unionist mode the other day, saying that Mr Salmond was a deluded fantasist who believes he is Nelson Mandela.

The chairman of the Scottish Tories, Grant Shapps, claimed Alex Salmond was trying to destroy democracy by threatening to vote down a Tory budget; an attempt, he insisted, "to undermine a government chosen by the British people".

Labour commentators have been accusing Mr Salmond of revealing his true elitist colours by ordering pink champagne during a lunch paid for by the New Statesman magazine. "Lah-de-dah" jeered Jim Murphy's campaign chief, Blair McDougall, suggesting that this was a scandal comparable to, well, Ed Miliband's two kitchens.

Then David Cameron at PMQs accused Mr Miliband of being "Alex Salmond's poodle". You could be forgiven for thinking that the former First Minister was still the leader of the SNP. He is not. He is a former political leader with a book to sell. Yesterday's man.

This obsession with Mr Salmond is bizarre on a whole range of levels, but it makes least sense of all in terms of political strategy. The Tories presumably hope to whip the Daily Mail brigade into a lather of righteous anger at a Scottish politician threatening to take over their green and pleasant land.

But anyone with an ounce of sense would realise that this is a stupid argument and strategically unsound. UK voters will soon discover that Nicola Sturgeon is the SNP leader, and she's very different.

Certainly, no one in Scotland believes that Mr Salmond is still in charge, and those Tory posters depicting him looking as if he has Mr Miliband in his pocket are of benefit only to the SNP. What are they trying to tell us? First, that the only party the SNP looks likely to deal with is the Labour Party, which is exactly what the SNP want people to believe.

In Scotland, Labour has been trying to suggest the opposite: that in some way the SNP and the Conservatives are in league with each other. But any notion that the SNP and the Tories could be remotely in the same bed together should have been dispelled by the extraordinary body language of Tory minister Anna Soubry on the Andrew Marr programme at the weekend.

She said she was "terrified" of everything Mr Salmond said and tried to edge away physically from him on the sofa as if he were an axe murderer.

But Grant Shapps surely killed the Nat-Tory alliance stone dead when he condemned Mr Salmond for suggesting that the SNP might vote down a Tory government. Shock. Horror. End of democracy as we know it.

Is he seriously suggesting that Scottish MPs have no right to vote against a Tory government's budget? That is what Mr Salmond was talking about in his interview.

What Mr Shapps presumably means is that, if the Tories are the largest party, they should be allowed to form the government and the "little" parties should jolly well accept it.

But this is wrong. It has never been the case in Westminster that the biggest party gets to be the government, as Labour also keep saying. Whoever commands a majority of MPs in the House of Commons gets to be the government, and that needn't involve the largest party.

Look at the Guardian's latest poll projection on the outcome in May: Conservatives, 274 seats; Labour, 271; SNP, 53; Liberal Democrats, 26; Ukip, four; Greens, one. If this happens, David Cameron may try to form a minority government with the LibDems and Ukip, but his budget would certainly be voted down, letting in Mr Miliband.

The scenario Alex Salmond outlined to the New Statesman was perfectly democratic; indeed, voting down the Tory budget would be the only course that the SNP could honourably pursue.

The SNP have repeatedly ruled out helping sustain any Conservative government in office, not least because they are opposed to Tory austerity. If a minority Tory PM put forward a budget like last week's, the SNP would be morally bound to vote against it, bringing down the government. It really is as simple as that.

It would then be for Mr Miliband as leader of the next largest party to put forward his own Queen's Speech and invite parliament to support it, which the SNP inevitably would, if only to keep the Tories out. This would ensure that Mr Miliband became PM.

Some have suggested that Mr Miliband might refuse to be prime minister if it was on the basis of support from the SNP. But that is a ridiculous proposition. He cannot refuse to form a government after winning a confidence vote in the Commons on his legislative programme.

Anyway, his MPs would never let him walk away. This doesn't even require a semi-formal "confidence and supply" arrangement with the SNP. It just needs the Nationalists to consistently vote to keep the Conservatives out of power; to "lock them out" as Mr Salmond put it.

But wouldn't Mr Cameron call another general election if his budget was voted down by the SNP? He couldn't. Thanks to the 2011 Fixed Term Parliament Act another election requires a two-thirds super majority in the Commons, and the other parties wouldn't vote for a dissolution.

After Mr Cameron fell the Queen would summon Mr Miliband and invite him to form a government The opposition leader would have 14 days to submit his own Queens Speech. Only if that failed to win a majority would there be another election.

Actually, my tip is for a Labour-LibDem minority government with SNP support. If you look at the respective programmes of the three parties there are remarkable similarities on austerity, Lords reform, home rule, minimum wage, EU membership, immigration and so on.

It would be quite possible for the three to agree a basic programme. The LibDems would be mad to go into alliance with the Tories again if they could be back with Labour. They worked very well with Labour in Holyrood.

Wouldn't the SNP ruin the party? Why should they? Mr Miliband holds over them the ultimate threat: a Tory government. He would dare Ms Sturgeon to vote down his Lib-Lab government and allow the Tories back in. It would be 1979 all over again, only much worse. Really, Ms Sturgeon would be the poodle, not Mr Miliband.

Also, she is not Mr Salmond. She doesn't relish opposition for opposition's sake. She would claim credit for the positive policies of a Labour government and say that the SNP had saved the UK from Tory austerity.

Some might argue that it would be better for the SNP for there to be a Tory government in Westminster because they are more unpopular than Labour. That is broadly true, though in recent polls Mr Miliband has been scoring more poorly than Mr Cameron.

But the point is that the SNP cannot allow themselves to be harbingers of another Tory term. If they have the numbers they have to vote with Labour. On present polling figures, Labour are home and dry even if the Tories are the largest party. Mr Miliband can't help becoming prime minister.