It appears, from what I’ve been watching on video, that Trump has long confused public sovereign debt with trade deficits, and is a mercantilist at heart. Which makes him a protectionist, too. Hence his earnest pushing of anti-immigrant demagoguery.
The fact that the Republican votership goes for this nonsense is not surprising to me. FREE TRADE was always a Democratic Party principle, not a Republican Party one. The GOP started out as the Whig Party redivivus, and Republicans were protectionists for their first hundred years. But at least since William Jennings Bryan, a strong protectionist streak has run down the yellow backs of populists in both parties. Most people don’t see the peace dividends of free trade. They only think of managing “their” advantages, while avoiding all talk of secondary and tertiary effects.
Trump is dangerous because he is a protectionist (as is Bernie Sanders), but (unlike Sanders) sometimes pretends to be free trade. The fact that America engages in massive mutual trade agreements rather than the less expensive and corrupting policy of unilateral free trade, plays into both Trump’s and Sanders’s hands. And Hillary Clinton’s, too.