I'm pretty sure its been said well before but the problem with annexation would be Ontario, Quebec, and BC.
In terms of the electoral college, the Maritimes and West would roughly balance eachother out (PEI would have to be merged with NS or NB since its too small), with Manitoba being a potential swing State. When you add in the demographic swing from Blue to Red States in America right now, its not that bad. It would just be physically extending New England and the American West.
However, throw in ON, QU, or BC and now you have around 40 votes that go hard left and want gay space communism. Its not coincidence that most of the retarded news you hear from Canada comes from these 3 places.