Is America an idea or a homeland? That question lies at the heart of this roller coaster of a presidential race. It’s about whether we Americans should continue to set our sights on global leadership and enforcing universal principles or instead hunker down and take care of our own. President Biden, in his speech on Wednesday explaining why he withdrew his candidacy, described America as “the most powerful idea in the history of the world.” In language echoing legacy Republicans like Ronald Reagan, Mr. Biden said that it was “an idea stronger than any army, bigger than any ocean, more...