While I do not know enough to speak on the issue of Canadian-American economic relations, I do know that the current NAFTA set up is keeping Mexico afloat.  For example, look at the economic differences between northern and southern Mexico.  The people in the south tend to be subsistence farmers and generally much poorer than the people of the north who are employed in factories that ship products across the border to the US.  The freedom with which good produced in Mexico can be transported into the American market, combined with the lower costs of production in Mexico give many corporations incentive to manufacture their goods south of the border.  Without this economic symbiosis, the large border towns of northern Mexico would cease to have reasons to exist.  There would be little economic incentive for American corporations to move their manufacturing, and a large market for Mexican agricultural products would be cut off.  The relationship between the United States and Mexico is vital to both nations' economies.  The cheaper products coming from Mexico allow us to increase our standard of living, while the money to pay for those products sent to Mexico allow them to increase theirs.  Economic cooperation between at least these two neighboring countries is necessary.  While a EU-like union may not be the best way to handle things, economic ties between the nations must be maintained or even strengthened.