DONALD TRUMP can rarely resist the urge to engage in name-calling. Typically his barbs are more juvenile than consequential. But his designation of Jerome Powell, chair of the Federal Reserve, as “Mr Too Late, a major loser” is no mere insult. It might be the harbinger of an assault on the central bank, and it is one more reason to worry about the American economy. When markets opened on April 21st, after a long Easter weekend, American stocks, Treasury bonds and the dollar all sharply declined—another example of the “sell America” trade.