THERE’S been a lot of talk of late about a coalition between Labour and the SNP after a hung general election.

Ed Balls is dead against it; Ed Miliband isn’t. But what about a coalition of Labour and the Conservatives? You what? Ed Miliband and David Cameron in the same government? Ed Balls and George Osborne running the economy? It would be manna for the Nats – all Nicola Sturgeon’s Christmases coming at once.

But believe it or not this idea is being seriously canvassed in some metropolitan media and political bubbles. The boss of the polling organisationYouGov, Peter Kellner, has been speculating in the current issue of Prospect magazine about the possibility of a grand coalition between Labour and the Tories.

The scenario is thus: after May 7, no-one wins. There is a hectic round of coalition-building with the “wee” parties like the SNP, Liberal Democrats, Greens, Ukip etc. Her Majesty the Queen holds a few dinner parties-from-hell in Buckingham Palace trying to get various party leaders to agree to form a government. When this fails, there is another general election, possibly in October.

However, after second general election, the situation is even worse, with the SNP and other upstarts cannibalising the electoral carcasses of the former “big beasts” – Labour and the Tories. Miliband and Cameron – or perhaps Yvette Cooper and Teresa May if the present leaders fall on their swords – get together and say: “For the sake of the country we must form the only stable government we can, which is a coalition of the centre ground against the madness of the Nationalist-Green-Ukip wreckers”.

Grand coalitions are quite common in countries with systems of proportional voting like Germany, though they are for obvious reasons very alien to the UK tradition of first past the post. Which doesn’t mean that, in extremis, the big parties here might not decide to join forces on a common programme.

What would the Labour-Tory programme be? Well, probably a referendum on Europe, another four years of austerity (“balancing the books” as Ed Miliband put it last week), Trident renewal, stricter controls on immigration, a home rule bill for Scotland, a constitutional convention on English Votes for English Laws. Labour and the Conservatives have been moving quite close on a number of these issues.

Every time a Labour minister like Yvette Cooper opens his or her mouth, they say they “got it wrong” on immigration. Labour supports the principle of Universal Credit to streamline the welfare system and reform housing benefit. Both Labour and the Tories are committed to legislation implementing the Smith Commission and broadly agree on the need to address the West Lothian Question.

They both also want to renew Britain’s nuclear “deterrent” in a dangerous world with the nasty Russian bears buzzing around the English Channel. Why shouldn’t Labour and the Tories get into bed if it looks as if the rise of the radical parties is making British democracy unstable? They joined forces in the war, didn’t they? Isn’t this the political equivalent?

No. A grand coalition is a very bad idea because it would be the grandest suicide note in history. It would confirm the worst suspicions of cynical voters that the old establishment parties are essentially interchangeable, and are so hostile to reform and change that they will do anything to defend the status quo.

The Greens, Ukip and the LibDems would love it. As for the SNP, it might seem hard to envisage Labour becoming more unpopular than it is in Scotland right now – but speculation about a grand coalition could certainly do the trick. The SNP would be even more cock-a-hoop than they are at the moment, if that is possible.

A devil’s bargain between Labour and the Conservatives would be infinitely worse for Scottish Labour’s image than all those pictures of them standing with the Tories in the Better Together campaign. At least that was on the broad issue of the constitution – a grand coalition would confirm the Nationalist claim that Labour and the Tories are essentially pro-austerity small “c” conservatives. And of course, irremediably “English”.

Actually, I think if there were a grand coalition in Westminster, Labour in Scotland might cut itself off entirely from the UK party and set itself up as a wholly Scottish Independent Labour Party, harking back to the tradition of the old ILP in Scotland. It is moving in this direction already.

The last time Britain had a grand coalition outside wartime was in 1931 when the Labour PM Ramsay MacDonald joined forces with the Tories in a national government during the Great Depression. That split Labour from top to bottom. This time it is more likely to be split laterally, with the Scottish party detaching from a more Blairite UK party.

Labour’s ideological contradictions were exposed again last week as the unreconstructed Blairites, Alan Milburn and Lord Hutton, served notice on Ed Miliband that his attempt to move left on issues like the NHS, taxation and freezing energy prices are going to be opposed to the bitter end. It’s been another terrible week for the UK party as it tries to get traction in the UK election campaign.

By contrast in Scotland, Labour’s hyperactive Scottish leader Jim Murphy was congratulating himself for forcing the SNP to introduce a moratorium on fracking, even though Labour abstained on it in Westminster. He is trying to outflank the SNP on the left by promising to use London’s mansion taxes to finance 1,000 more Scottish nurses, calling for a 50p top rate of tax and, most audacious of all, proposing the renationalisation of the railways.

His deputy Kezia Dugdale has had a couple of decent outings at First Minister’s Questions, making Nicola Sturgeon look uncomfortable over exam appeal charges last week. Labour are expecting Lord Ashcroft’s latest raft of constituency polls this week to show that the SNP’s poll lead isn’t translating into actual seats. New Jim may not be getting anywhere fast, but he deserves full marks for effort.

However, Labour’s London connection could still be his greatest handicap. Ed Miliband is actually less popular in Scotland than David Cameron, which is pretty astonishing when you consider it. On many policies – NHS, nationalisation, increasing the minimum wage – Miliband should appeal more to Scots than an Eton Tory who wants to make restoring fox hunting a key election pledge. But it doesn’t work like that.

What the old parties are facing is an uprising, not of “anti-politics” as it is called in Westminster, but anti-establishment politics. “They all look and sound the same” is the most corrosive charge of all and one the SNP believe will trump Jim Murphy’s claim that a vote for the SNP is a vote to let the Tories in. Scots voters seem to resent it even more when a Labour leader looks like part of the Westminster clique.

Politics has gone one step beyond. We are in a more plural electoral environment in which outsider parties can become mainstream very rapidly, as we saw in Greece last week with Syriza’s victory, and in the Scottish independence referendum. Jim Murphy is making the right moves in trying to make Labour more autonomous and left-wing. And some Scots will worry in May that a vote for the SNP is a vote to keep the Tories.

But Murphy can’t avoid the claim that whatever else it may be, a vote for Labour is still a vote for Labour.