This week, Bush, Harper and Calderon met for a "North American Summit" to discuss NAFTA. Each time the presidents of the US, Mexico and PM of Canada have all met over the years, they've been secretly planning to expand NAFTA into a North American Union.
All of this has been done with little discussion in Congress. But keep in mind, Congress was probably opposed to NAFTA, but then early in Bill's presidency, Congress approved of it.