Assuming the Republicans do well, it will mean that over the next two years both parties would be simultaneously trying to present themselves as at one and the same time being the government and being the opposition - trying to appeal to the instinct people have to back any incumbent, and the instinct other people have to throw out any incumbent.

There's a thing called "the luxury of opposition" - and neither party is going to be able to enjoy it.

If you has a different electoral system, this would be an opportunity for a third party to make a breakthrough.