Ed Miliband's response to Nicola Sturgeon's appeal for Labour and the SNP to form an anti-Tory alliance ("It's a no, I'm afraid") has been presented by some as a rejection of any possible deal. But his preceding words make it clear that he was again only ruling out a formal coalition between the two parties: "I have fought the Tories all my life. We have profound differences. That why I'm not going to have a coalition with the SNP. I'm not going to put at risk the unity of the United Kingdom. It's a no, I'm afraid."

This leaves open the possibility of a confidence and supply arrangement between Labour and the SNP under which the latter would agree to support the former in confidence votes ("confidence") and Budgets ("supply"). Sturgeon said this morning: "I suppose you have to ask Ed Miliband what he meant. He’s ruled out a coalition, I’ve always said there isn’t going to be a formal coalition between Labour and the SNP. But let's just think about three weeks from today, 8th of May, if Ed Miliband and the SNP between us have more MPs than David Cameron has, is Ed Miliband really saying that he would stand and watch David Cameron get back into Downing St? If that is Ed Miliband’s position then let him come out and say that. And people across Scotland and across the UK can draw their own conclusions."

Ahead of parliament almost certainly being hung again, there are many on the left and the right who argue that a Labour-SNP deal is inevitable. Some in the former camp even argue that the party should welcome the chance to forge a new "progressive alliance". But as several Labour sources have recently pointed out to me, this ignores how the party could call the SNP's bluff in a hung parliament. The nationalists' commitment to vote down any Conservative-led government has left them with none of the bargaining power that the Lib Dems enjoyed in 2010. If Labour and the SNP have enough seats between them for a majority, Miliband could simply challenge the party to vote in favour of his first Queen's Speech, which would be filled with the kind of left-wing populist measures that Sturgeon could not oppose (an energy price freeze, a cap on rent increases, tougher banking regulation, a crackdown on zero-hours contracts, a housebuilding programme and further Scottish devolution). This would avoid the need for Labour to do a deal with the SNP and risk condemning Scottish Labour to permanent irrelevance while also alienating English voters. It would also prevent Miliband from having to compromise on deficit reduction, Trident or any other contentious matters.

The risk remains that the SNP does not blink and either abstains or votes against a Labour Queen's Speech, potentially triggering a no confidence vote and a second election (if no government can be formed within 14 days) that Miliband may not win. But Labour would be able to warn of the danger of the Tories returning to power (as they did when the SNP helped bring down the Callaghan government in 1979). The alternative scenario - that the SNP voluntarily puts in Labour in power - deserves more consideration than it is currently receiving. And it is path that Miliband is most likely to pursue if he falls short on 8 May.