Supporters see it as the best hope for escape from economic stagnation, a boost for trade and investment, a boon for employment, a lift for standards of living. Critics counter that it will strike a mortal blow at entire sectors of U.S., Canadian and Mexican industry, idling tens of thousands of workers whose jobs will move elsewhere, never to return. Europeans and Asians fret that it may accelerate a division of the world into giant protectionist trading blocs lurking behind new walls of tariffs and bureaucratic restrictions.
The subject of these conflicting visions is an edifice of daring scope and complexity,...