In Rhode Island on Wednesday, Senator Jack Reed — a 76-year-old West Point graduate and Harvard-trained lawyer whose father once mopped school floors — turned back two progressive challengers to claim his party's nomination for a sixth Senate term. The contest illuminated a tension that runs through the Democratic Party broadly: between the patient incrementalism of the institutional left and the urgent moral demands of its insurgent wing. Reed's commanding financial advantage and three decades of accumulated political capital proved, once again, that durability is its own form of power.