When President-elect Donald Trump replaces Barack Obama on January 20, the Democratic Party will find itself more removed from power than at almost any point since the party’s creation. Scorned by the same voters who once embraced the New Deal, built the Great Society, and put their hope in the nation’s first black president, Democrats are now locked out of power in Washington and out of two-thirds of state legislative chambers across the country. Simply put, Democrats’ once vaunted coalition of the ascendant — younger, multiethnic, educated, and urban — failed them in 2016, and in 2014 and 2010 before...