Something in the Democratic Party’s subconscious knows that in times of change it should go to Chicago.It’s the city where, in 1896, a 36-year-old William Jennings Bryan sent convention delegates into raptures by railing against Gilded Age plutocrats and urging Democrats to reconnect with their populist roots. It’s where Franklin Roosevelt announced the coming of a New Deal in 1932, starting a political revolution that pushed the Republican Party to the verge of extinction. It’s where the Roosevelt coalition ripped itself apart in 1968, with protesters and students brawling in the streets and delegates at one another’s throats. It’s where...