On the most recent Andrew Marr show the Labour leader said there would be no confidence and supply deal with the SNP, and earlier ruled out a coalition, but unless Labour and the Lib Dems can reach a majority Labour might have to rely on the SNP if it wants to get into power.
Let’s assume that in private Ed Miliband is willing to work with the SNP, or at least let them help him get into power, how many seats does Labour need to gain?
Recent polls suggest that the SNP are set to take almost every seat in Scotland. Whether this is true or not will be seen on election night, but for the purpose of the parties working together, Labour and the SNP combined currently have 47 seats. After the election the majority of them could be SNP. Furthermore, let’s assume out of the Liberal Democrat seats the SNP take a majority (at least six) and the Conservatives hold their single seat.
In total the SNP-Labour bloc will have a minimum 53 seats north of the border, a net increase of at least six seats, more depending on how many the party take from the Liberal Democrats.
In 2010 Labour won 258 seats. With the assumption that Labour and the SNP could work together - and that their seats north of the border count as the same - any losses north of the border will be made up for by SNP gains. A Labour minority government - propped up by the SNP - will need to gain support primarily from England.
Let’s add in the six seats that the SNP gain from the Lib Dems (at a minimum based on the falling support of the Lib Dems and growth of the SNP). This takes the Labour total to 264.
The magic number for a majority is 326, or 323 assuming Sinn Fein do not take their seats. As a result Labour will need to gain 59 seats, perhaps a few less depending on how many seats the Liberal Democrats lose in Scotland to the SNP.
If Labour gain 50-60 or so marginal seats in England and Wales then a Labour minority government with SNP backing looks possible, depending on Labour’s willingness to work with the nationalists even if it is on a vote by vote basis. This number might be even smaller depending on the willingness of Plaid and the Greens to prop up a minority administration.
Recent polling has suggested that Labour is making gains in marginals, but whether it's enough is yet to be seen.