That will be a LONG time coming, if ever. Canada and the USA are drifting together on enough sociopolitical issues/outlooks where such a possibility (though remote) does exist. But the Canadians don't need to be part of the USA to be successful. Mexico is pretty much always in tight straights, and would benefit from such a union in a number of ways, but the cultural differences would make it even more difficult and less likely.
Final verdict = please don't hold your breath waiting for the USA to encompass the whole continent.
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