During his first term as president, Dwight Eisenhower turned to his press secretary, Jim Hagerty, and said, “If the right wing wants a fight, they’re going to get it. If they want to leave the Republican Party and form a third party, that’s their business, but before I end up, either this Republican Party will reflect progressivism, or I won’t be there with them anymore.” Eisenhower wrote in his diary that he might even have to form a third party himself, and he would dedicate it to “the Middle Way” — his term for the political centrism that he favored. This could be a potent theme for today’s...