Another name for this may be the 2nd American Revolution. Not many American patriots are going to stand by and let our constitution get watered down to the lowest common denominator. If we join Mexico and Canada the Bill of Rights will be gone.
The way I see it, the only country we have any business talking merger with is Canada, and then only if the Bill of Rights is considered supreme over the entire union. That includes, of course, the Second Amendment. Forget Mexico. Imagine a common defense where some of the troops would demand "mordida" from the people they're protecting. Sheesh!