This is written for UK readers, who will know who all the parties are. However you do not need to know the details to get the main points.
How do you translate seats into governments? So many parties, so many variables. Here is a suggestion, based on the idea that the number of seats that the big two are likely to get is much more uncertain than for the other parties. Then split those other parties into three groups: the ‘left bloc’ (mainly SNP, but also Plaid, Greens, Galloway and SDLP), the LibDems, and a ‘right bloc’ (NI Unionists plus UKIP). If Sinn Fein win 5 seats which they do not take up, there are 645 seats to play for, and a total of 323 seats gets you a majority.
Assume the left block gets 58 (50 SNP, 3 Plaid, 3 SDLP, 1 Green, 1 Galloway), the LibDems get 24, and the right block get 13 (e.g. 3 UKIP). That is a total of 95, leaving 550 to divide between Labour and the Conservatives. We can now split the result into a two dimensional set of possibilities depending on how the Labour/Conservative battle goes:
1) Con 323+ (Lab 227-) seats - Simple Conservative government
2) Con between 310 and 322 seats (Lab 240-228) - Coalition continues.
If you add the right bloc to 310 you get 323. In this case the Conservatives could shun the LibDems, but I suspect they will prefer to continue to work with the LibDems than having to rely on the right bloc, and the LibDems would find the attractions of continuing in government too strong to say no.
3) Con between 286 and 309 seats (Lab 264-241) - LibDems decide.
In this situation the LibDems have indicated they would ‘talk to the Conservatives first’, but in principle they could go either way: see below.
4) Lab between 265 and 298 (Con 285-252) - Labour SNP understanding.
265 plus the left bloc of 58 gets to 323. Would the LibDems get a look in? There is a possibility near the Lab 265 mark, where Labour might want to avoid the chance of one of the smaller left block parties causing difficulties. But it is not clear why Labour would want a coalition with the LibDems in this case, rather than something similar to their SNP understanding.
5) Labour between 299 and 322 (Con 251-228) - Labour choice
Labour would still have the option of an SNP arrangement, but it could alternatively form a coalition, or make an arrangement, with the LibDems. Some suspect Labour would find the politics of not being dependent on the SNP attractive, but they would probably want a new LibDem leader in exchange.
6) Lab 323+ seats - Simple Labour government.
At the moment the polls and models are suggesting we are in Labour-SNP territory, but only just, and things could easily slip into the ‘LibDems decide’ zone, with other outcomes still quite possible. If the LibDems do have to decide between Cameron and Miliband, things get very interesting.
There seems little doubt that Clegg and some other senior LibDems would favour continuing with the current coalition: the fact that Clegg has indicated he will talk to the largest party first indicates that, because in itself talking to the largest party makes little sense when the left block is so much larger than the right block. However most of his party members would more naturally favour the opposite arrangement. Furthermore, LibDem policies are generally closer to Labour than the Conservatives.
There are plenty of superficial sound bites that will be brought into play by the LibDem leadership to favour choosing the Conservatives: besides largest party talk, there is also the line of not wanting to be associated with a party that wants to break up the union (also largely nonsense). But what LibDem members really need to think about is the survival of their party if they continue with the current coalition when they have perfectly feasible alternatives. Imagine, for example, if the referendum went against continuing EU membership. Even if it did not, imagine the next two years when uncertainty about EU membership held back investment, and growth faltered under renewed austerity. Imagine the NHS finally collapsing from lack of funding. The Conservative party would recover from all those things, but the LibDems would always be seen as the party that chose to let this happen. I’m afraid I do not know enough about LibDem internal decision making to know how far the party would in practice be able to overrule its leadership in the days after the election. (If you do, please comment or email.) If they cannot, or chose not to, this zone merges with ‘coalition continues’.
Of course the number of seats obtained by the LibDems, and other minor parties, are uncertain, although less so than the Con/Lab split. However realistic changes to these numbers just move the location of these various zones, rather than change the zones themselves. The zone margins are also probably not precise: for example it is conceivable that one of the major two would prefer a LibDem coalition to having a majority of only 1.
One final point about the role of the SNP. If the SNP seat count was much smaller (which seems very unlikely to happen), we would have a symmetrical position. In essence it is a three party model, with the LibDem position being ‘forced’ on the boundaries because the largest party has the option of instead going with the minor parties. A large SNP block creates an additional zone, where Labour can only govern with SNP support. Now suppose you are indifferent between the policies of Labour and the SNP, but are inclined to vote SNP because you think that would give Scotland more influence. Is there a downside? There would seem to be two.
The first concern would be that some English voters might not vote Labour because they did not want Scotland to have this additional influence. You could see Conservative claims of Labour-SNP chaos as code for this. The second is that it may deprive Labour of being the largest party. However that does not matter in any constitutional sense here, because the way I have set up the problem each minor party (in terms of seats) will only form a coalition with one of the major two. (The DUP might be an exception to this rule.) It only matters to the extent that being the largest party in terms of seats might influence the LibDems choice! Interesting times.