The SNP is on the rise, easily the UK’s third largest party by membership and holding the momentum of what, by any standards, was a successful referendum campaign even in defeat.
Its morale and confidence are so high, that it has scared the main GB parties into playing games around potential deals with the SNP. Perhaps they (particularly the Conservatives) feel this suits them particularly, but the practical fact is the SNP has an extraordinarily weak hand in any post-election negotiations.
Firstly, and understandably for electoral reasons (but probably foolishly in practice), the SNP has ruled out any deal whatsoever with the Conservatives. Practically, this leaves it with no choice but to back Labour – already a weak hand.
Secondly, this actually leaves the practical choice for the SNP of backing a stable Labour government (depending on the numbers), or creating instability. The latter may be tempting for a party which may feel it would gain from this, but in fact the inevitable result would be an early UK election.
Such an early UK election would not suit the SNP at all. It would in fact fully endorse Scottish Labour’s contention that the only way to ensure (as much as possible) the Conservatives stay out is to vote Labour, almost certainly costing the SNP seats; and, of course, if the Conservatives won an election forced by unreasonable SNP demands, they would be blamed for it, potentially wrecking their entire strategy for decades.
In other words, the SNP, even with all 59 Scottish seats, cannot afford to endorse a Conservative-led government and cannot afford to force an early election. It has no cards to play at all.