You will never elect another Republican President, ever, ever again.
Nah, parties would just adapt to the new political reality.
If Canada and the US ever became one single state, I imagine that it would have to become a highly decentralized one. Probably the only thing the two sides might agree on would be a common passport and citizenship. This would mean a highly decentralized government with the individual constituent states operating almost like sovereign states in their own right except for the free movement of people. But that might not be a bag thing.
Oh, and they might agree on a common currency and a small military. However, I could imagine the US states deciding to supplement that with their own state militias and Canada being more than happy with the Federal military force.
But again, not necessarily a bad thing. Canada would save money on the military by amalgamating its military force with the US one, and states that think such a small military ain't enough could spend as much as they want above that.
To prrvent a problem like the Euro thought, we might need to impose strict limits on how much a state government could borrow. Your state wants a militia, by all means, but do it within your existing budget.