Popular national figureheads like Bernie Sanders and Elizabeth Warren, a roster of organizers trained by the grueling 2016 primaries, a compelling electoral rationale, a well of ideas to draw upon, and energy from a rising generation of young voters—not since the Depression have so many factors come together in the left’s favor. Debates about the direction of the party that were stifled by the need to rally around Obama in the Great Recession are finally bursting into the open. These are arguments Democrats need to have—and they are arguments the left should win.