During the 1980s, Ronald Reagan bestrode the political landscape like a colossus. After two Reagan landslides, the Democratic Party was on its heels, having lost one-in-four of their supporters to the popular Republican. Democrats took pains to study the so-called “Reagan Democrats” and learn what made them tick. Leading the charge was the Democratic Leadership Council, which advocated that the party abandon its Big Government, New Deal-like approach to problems and support a strong national defense and middle-class tax cuts. Bill Clinton, Al Gore and Joe Biden were among its prominent members who liked to call themselves “New Democrats.” Reagan...