The
Guardian's predictions yesterday suggested that Lab+SNP would fall in that odd gray section of Westminister, created by six Sinn Fein MPs who never take their seats, with 323 seats overall. Enough to survive votes of confidence but not strictly speaking holding a majority.
The SNP have stated a preference for a confidence and supply arrangement, which would avoid the coalition trap effect that's expected to halve the Lib Dems' seats(or worse) in May.
Formal coalition or confidence and supply, that margin is very thin and extremely vulnerable to rebelling MPs. Whether the premiership remains with the incumbent or passes to the challenger, he will have to horse-trade with everyone just to avoid another general election, never mind run the country.
The one possibility the Guardian doesn't mention is a grand coalition of the Labour and Conservative parties. They would command 547 of 650 seats, based on the Guardian's numbers. Rebellious MPs could be rife and the government would still be able to pass legislation - provided the party leaders agree and are able to stave off leadership challengers(even then, the coalition could still endure). I'm not sure this outcome is likely, but hey, five years ago a lot of people wouldn't have believed the Libs would go into coalition with the Tories and then prop them up for a whole five years.
Coupled with an electoral pact not to contest the other's currently held seats, such an arrangement could hang on for the next 10 or 15 years, particularly as it looks like there'll be at least three mid-weight parties fighting over the voters lost by the duopoly.
One of the most
inconceivable result of such a grand coalition is who would become the Leader of Her Majesty's
Loyal Opposition - the SNP leader in the Commons!
The other possible outlier is some kind of unrefusable offer from the Conservatives to the SNP - Sturgeon would find it mighty hard to explain turning down full home rule minus foreign affairs and defence(the original Devo Max) in exchange for confidence and supply in the rUK, for instance. Various other issues could enter this deal, such as Trident renewal and electoral reform of the UK on a federal level.