According to at least one election forecast (at the time of writing this post) the only Government with a majority that could be formed after the May 7th election would be a Labour led one, but one that would involve Labour having some sort of agreement with both the Liberal Democrats and the Scottish National Party (SNP). But if you think that this would mean it is open season for the SNP to make Labour bow to a lot of demands, think again! That's because the SNP would have no credible alternative to providing Labour with a 'supply and confidence' majority. So in order to gain any political concessions the SNP would have to ensure that demands were both moderate and not incompatible with Labour's overall programme. They could do best by allying with existing interest groups within the Labour Party who favour certain priorities.
This emphasises the importance and usefulness of renewable energy in party political terms as something Labour could take on board and demonstrate that the SNP is gaining political benefits from any alliance with Labour. See my earlier post http://realfeed-intariffs.blogspot.co.uk/2014/12/renewable-energy-near-top-of-snp.html
In theory the Government's existence would be in the hands of the SNP. But in practice the SNP would face severe damage (and lose many seats it is likely to gain in May 2015) at the following election if its actions resulted in the actuality or greater prospect of a Conservative Government at Westminster. They would simply confirm Labour propaganda about the SNP letting the Tories in to power at Westminster.
However, unless the Conservatives make significant gains by May 2015 on what the polls are at the moment, a Labour Government with pacts with the Lib Dems and SNP seems the most likely outcome. Many commentators seem to be saying that so long as the Conservatives emerge as the largest party, then they will form the Government. That's not necessarily the case at all. The Conservatives need to be able to mobilise the largest block of MPs, which is not the same as being the largest party. The SNP will not support a Conservative Government, and the SNP need to be seen to be backing a Labour administration as an alternative.
If Labour, together with the SNP (and others such as PC, SDLP and Greens) achieves a larger number of seats than the Tories, DUP (and maybe UKIP) then the Lib Dems are likely to back a Labour minority government. The Lib Dems have a preference for Labour. See, for example: http://labourlist.org/2014/10/further-poll-finds-lib-dem-members-would-prefer-a-partnership-with-labour-over-tories/. The Lib Dems would, if Conservatives were the largest party, follow convention and talk to them first about forming a Government. But if the Conservatives end up with less than 300 seats that could be a very short conversation. Instead they would have more serious talks with Labour.
Imagine the following scenario of parties and seats:
Lib Dems 30
Sinn Fein 5
This is a variation on the numbers coming out of the website: http://www.electionforecast.co.uk/
which in fact at the time of writing gives Labour a seat lead. Even this calculation by Hanretty et al assumes that the Conservatives will be doing better in the polls at the time of the election than they are now. - so the seat scenario I make above assumes that there is a significant swing towards the Conservatives between now (Jan 16th) and the election to give them a lead of 2.5- 3 per cent in votes cast over Labour.
At least 323 seats are needed for a practical majority in the Commons since Sinn Fein do not attend the Commons votes (otherwise it would be 326 out of 651 MPs). Using my scenario the Labour 'bloc' (Lab plus SNP, PC, Green, SDLP) will be 310. On the other hand the Tory bloc, even if we were to include DUP and UKIP (the latter being a questionable ally for the Conservatives in practice since they would repel the Lib Dems) would be only 301. Who would the Lib Dems go with in that scenario? Well the arithmetic favours going with the Labour bloc since that gives a more secure majority anyway, but the party political preferences also make Labour a more attractive option.
Note I am talking about supply and confidence pacts here as the mode of forming and sustaining a Government (perhaps for no more than 1 year). Coalitions seem much less likely (and not at all involving the SNP).