Since we have the biggest economy we should have the say so, if they want to join with us. You can't have different laws because you drove a few feet over an imaginary line.
Im not saying what should be Im saying what I believe would be. The democratic process involved would be filled with nationalistic feelings. Neither Canada nor mexico would want to be seen as being swallowed up by the United States. They would want a new constitution. I cant see them not wanting one. Deal breaker. Not that I believe the merging will ever happen.