Should the US propose annexation with Canada?

[Frodo]

Yes, but I prefer the gradualist approach, uniting with Canada by increments (NAFTA is the first step) such that by the time annexation is proposed, it would be a mere formality.  

[Frodo]

Yes, but I prefer the gradualist approach, uniting with Canada by increments (NAFTA is the first step) such that by the time annexation is proposed, it would be a mere formality.  

Why? What makes the US+Canada (which would be larger than Russia by land area) a more geographically ideal political unit than either of them on their own?

This. Also, it would eliminate a long and porous overland border, replacing it with a maritime border covering the Pacific, Atlantic and Arctic coastlines that can be more easily patrolled by the US Navy and Coast Guard.  It would also enable us to more direct control over who comes in (and out) of Canada's ports, complementing our national security.