Any movement with the power to drive voters at the numbers you need to win is always going to feature extremism, dumb kids, and alienating self-righteousness from the fringes, as well as belligerently insistent groups whose job it is to push a party further than it wants to go. But a party that wants to lead and act — that has at its core a set of principles it is determined to lead and act on — should be capable of metabolizing these dynamics.