Without naming Republican presidential nominee Donald Trump, former President George W. Bush delivered an incisive critique of his policies of “isolationism, nativism and protectionism” at a private fundraiser in Cincinnati on Tuesday for Ohio Sen. Rob Portman, according to four people who attended.

Mr. Trump has broken with GOP orthodoxy and pushed an “America First” platform that calls for renegotiating trade deals, reconsidering longstanding military alliances, and curbing immigration by Muslims and people from countries...