Miliband turns his back on a centre-left alliance

Ed Miliband may not win the next election, but he plainly believes he will. The Labour leader put in a confident and cheerful performance on the Marr show yesterday – easily the best TV outing we've seen from him. But during the course of it he also sabotaged his own electoral advantage and consigned himself to minority government.

Despite his relaxed demeanour, the negative Tory campaigning has clearly spooked him. There are signs the Tory warnings about the involvement of the SNP in government is cutting through to some voters in England – particularly Ukip voters being urged to returned to the fold. Clearly, they scared Labour HQ enough to make them rule out a confidence-and-supply arrangement with Nicola Sturgeon's party.

Miliband said:

"I want to be clear about this. No coalitions, no tie-ins … I've said no deals. I am not doing deals with the Scottish National party. I'm not interested in deals, no … The way the House of Commons works is that we want to put our Queen's speech before the House of Commons and the other parties will vote."

Miliband and Sturgeon are mastering the art of pretending that they've said something for ages when in fact they are announcing it. Miliband had not previously said no deals – he said no coalition. Until yesterday morning, he had refused to rule out a looser arrangement such as confidence-and-supply. Now he is extending the language to include seemingly any arrangement with the SNP.

Miliband evidently plans to call the SNP's bluff. He'll put forward a Queen's Speech, then dare the nationalists not to support it and be known forevermore as the party which allowed the Tories back into government. It'll work. The SNP will support it. And what then? Miliband will run a minority government with nationalists on the outside able to undermine it whenever they like. Miliband's 'no deal' is a fiction – in a minority government you'd better be prepared to work with other parties or it will be time to pack your bags. You can either do that in an organised, structured way or leave yourself open to external forces on every vote. Miliband has deflated his own life raft and laughed about it as he does it. Barring a last-minute Labour surge, he has consigned himself to heading a weak government.

Not only that, but he is making it much harder for him to form a government. David Cameron can go to the palace with a coalition with the Lib Dems and probably (barring a Lib Dem rebellion over it) the DUP in his pocket. Miliband's refusal to work with the SNP means he just lost a 50-strong voting block to make his case for government. All he has now are a smattering of smaller parties and the Lib Dems – whose leader is making it increasingly clear he prefers working with the Tories.

But Miliband's actions aren't just damaging to his own political prospects. They are also extremely damaging to the Union and to the British left in general.

He has signed up to the Tory-mandated SNP response, which is to block them from government. The wave of support for the SNP will not just be ignored by Westminster. It will actively shutter-up the windows and conspire to prevent this democratic will finding a voice in government. There could be no better demonstration of the SNP's argument that Westminster does not respect Scotland and will never reflect its will. Miliband's decision makes the break-up of the Union much more likely. The Tories have won this battle. He blinked. Those who want to preserve the Union will pay the price for that.

Miliband has also shattered ay prospect of centre-left unity. As usual, the Labour party's primary enemy is not the Tories, but fellow travellers on the centre-left.

As the main parties suffered a breakdown in support, an opportunity arose for a divided left to get over its electoral disadvantage. This posed an extraordinary threat to the Conservative party. Previously, it could command almost all right-wing support, while Britain's majority centre-left vote was split among Labour, Lib Dems, nationalists and lots of smaller socialist and environmental parties. With neither Labour nor the Tories commanding a majority, this splintered left-wing vote could reassert itself. There was an opportunity for parties with similar policy platforms – like Labour, the Lib Dems and the SNP say – to make a convincing case for government on the basis of political compatibility.

That now seems a naive dream.

Lib Dem leader Nick Clegg said he would not prop up any government "held hostage" by the SNP or Ukip. It sounds like equidistance, but it is nothing of the sort. Ukip will get one or two MPs. The SNP will probably get about 50. They are not comparable. Actually, the only broadly comparable party on the right would have been the DUP and Clegg did not rule out entering a Tory coalition with them.

He then reaffirmed that he would try to form a government with the party with the "biggest mandate". It's not clear whether that means share of the popular vote or number of seats, but either way it is nonsense.

The fact a few thousand more people voted one way or another means very little. Much more important is the fact that voters have lent their support to parties with complementary or compatible policy agenda. Votes for the Lib Dems, Labour and the SNP may have differences, but they are clearly more similar than a vote for the Tories, Ukip or the DUP. These are progressive votes – a slower approach to deficit reduction with protections for the vulnerable, the safeguarding of human rights law, support for the EU, and a social vision which is more in tune with the centre-left than the Tories.

Clegg's statements ruled out qualitative rather than quantitative coalition formation. He views it as a numbers game based exclusively on the chief political brands – Tories and Labour – not a nuanced sense of why people vote and what broader political allegiances they are expressing with that vote. It is a false and simple-minded argument, and one which in this election is uniquely beneficial to the Conservative party.

His comments yesterday showed Miliband now supports that position. He has thrown the opportunity for a left-wing grouping in Westminster to the wind. He has allowed David Cameron and Clegg to call the shots and limit his options. For all his smiles, it was a ruinous weekend for the Labour leader.