The art of leadership is a serious matter. One must not lag behind a movement, because to do so is to become isolated from the masses. But one must not rush ahead, for to rush ahead is to lose contact with the masses. He who wishes to lead a movement and at the same time keep touch with the vast masses, must conduct a fight on two fronts, against those who lag behind and against those who rush ahead.

Josef Stalin (1879-1953) Georgian revolutionary and Soviet dictator

Leninism, Vol. 2 (1926) [tr. Paul (1933)]
    (Source)

Often elided, "He who wishes to lead a movement must conduct a fight on two fronts, against those who lag behind and against those who rush ahead."