I've got no problem with Mexico and Canada as joining the US as states no. 51 and 52...but just as states. We already have a Union.
Obviously the need for oil (Canada's and Mexico's) a big part of this direction, as well as having more direct control/cooperation with anti-terror interdiction in the entire Western Hemisphere.
You would give these 2 leftist powers 4, count em, 4 Senators....I think not....
Same way we cant make PR the 51st state. To many socialists.