There is no scenario under which withdrawing from NAFTA would benefit the United States economically.
The so-called protectionism being touted by President Trump and his supporters is little more than picking the pockets of U.S. consumers. Note, however, that much of the goods imported from Mexico consists not of immediately consumable goods, but of producer goods (e.g., petroleum, automobile parts, and components of a vast array of other manufactured goods) that help to make U.S. goods better and cheaper than they otherwise would be. The Trumpistas suppose that exports are a benefit and imports a regrettable thing [that] ought to be reduced as much as possible. In this regard, they have matters upside down: imports are what Americans value; exports are directly or indirectly only a means of importing the valuable goods. If you doubt this claim, simply imagine what would be the case if Americans regularly sent vast quantities of goods abroad and got back no goods at all. This situation would give rise to an infinitely positive balance of trade—and amount to an economic disaster. Sad to say, the Trump forces have infused new life into mercantilist fallacies that were debunked centuries ago by Adam Smith, David Ricardo, and a host of economists since their day. It is sad to contemplate how many voters prefer picking their neighbors’ pockets to honestly earning their own way in open competitive markets.
Ben Zycher rightly deplores the some policies of the Union of Concerned Scientists. (By the way, whenever I encounter the name of this group I wonder if there is somewhere a Union of Unconcerned Scientists.)