So in order to command a majority in the House this "progressive" coalition would have to be expanded to include some combination of the SNP (6), Plaid (3), Green Party (1), Alliance Party (1) and SDLP (2) to reach the magic figure of 326.
Some argue that because the five Sinn Fein MPs refuse to take the oath to the Crown that would allow them to take their seats, the real figure needed to command a majority is lower than that. This may be the case, though it is probably worth hesitating before you give Sinn Fein the balance of power.
But it still means that an arrangement between Labour and the Liberal Democrats would have to be expanded to command a majority.
So how does Labour treat the smaller parties?
Earlier today Alex Salmond, the leader of the SNP, called on the Lib Dems to join a "progressive alliance" involving Labour, his party and Plaid Cymru as an alternative to a deal with the Tories.
In reply, Labour issued a statement saying they were not in discussion with the SNP and describing the initiative as "a desperate attempt by Alex Salmond to make himself look relevant".
As Peter Hoskin says:
Well, that may or may not be true. But given how the Labour leadership has all the cards stacked against it, it wouldn't hurt them to be a little more circumspect over the next few days.