We have the weird situation that whichever party gets elected tomorrow is quite likely to damage themselves in the medium term by being elected.
It’s hard to see how either a Labour or Conservative led government will stay together and functional for five years.
From the Tories’ perspective there’s a strong anti-Tory alliance who will block them at every opportunity, and from Labour’s perspective there’s a bunch of previously loyal now unruly Scots trying to hold them hostage. Polls suggest that Labour are likely to win fewer seats than the Tories but they are more likely to be able to form a government, which will make a lot of people question its legitimacy, and if it then gets sunk a few years (or months…) later by the SNP withdrawing support then that will reinforce public opinion that they should never have been in government in the first place.
It seems almost certain that Labour can’t win both this election and the next one, which might not be very far away, so winning this one would be a pyrrhic victory.
On the other hand… as the outcome of this election is likely to be unprecedently unusual, especially with the SNP holding significant power, we might have the public backing proportional representation by 2020. In which case the main parties will become a lot less main.