One of the most stalwart advocates of free trade was Karl Marx:
“Generally speaking, the Protective system in these days is conservative, while the Free Trade system works destructively. It breaks up old nationalities and carries antagonism of proletariat and bourgeoisie to the uttermost point. In a word, the Free Trade system hastens the Social Revolution. In this revolutionary sense alone, gentlemen, I am in favor of Free Trade.”
And Marx was right with that quote. Free trade is destructive and it does work to break up nationalities and national borders.
That's why the first step toward a unified Europe was free trade and the EEC, and then on to the destruction of national sovereignty and the grand EU.
And free trade is the first step to break up nationalities and national borders in the Western Hemisphere, and then on to some sort of North American Unionm, or whatever.
Large numbers of Americans who think they're just supporting free trade like they learned in college are unwittingly serving the purpose of those who do want to see an end to US sovereingty and the creation of a larger, borderless hemisphere and world.