American politics is a famously contentious theater, especially today. But the vast majority of liberals, conservatives, and Washington journalists all seem to agree that “extremism” is appalling and should be eradicated.

Yet, the meaning of the term is as prey to ideological dispute as are such holy words in our political lexicon as “freedom” and “rights.” Conservatives call Bill de Blasio, who is about to be elected mayor of the nation’s largest city a “left-wing extremist,” while liberals counter that anyone with ties to the Tea Party is an “extreme right-winger.” “Extremist” is the description of choice for fundamentalists of any religion; unless, of course, you belong to one of the faiths being gored. In that case, it’s only traditionalists from the other religions who are “extreme.” “Community,” the literary critic Raymond Williams once observed, “seems never to be used unfavorably.” But no one has a kind word to say for extremism or extremists.       

Well, let me be the first. Sometimes, those who take an inflexible, radical position hasten a purpose that years later is widely hailed as legitimate and just. Extremism is the coin of conviction, whether virtuous or malign. It forces middle-roaders to crush the disrupter or adapt....