Hillary Rodham Clinton was asked this summer if she would describe herself as a “liberal.” The Democratic front-runner shied away, saying the “word” — noticeably not using the word — has taken on a connotation that “describes big government. “I prefer the word ‘progressive,’” she said. It has a “real American meaning.” Then she expanded the term to “modern progressive,” and, finally, clarified that she was a “modern American progressive.” These are heady days for Democrats. The party is favored by almost all measures in the coming presidential contest. But while Democrats are emboldened, they remain wary of the term...