The headlines looked like something left over from Election Day, but they betokened a strictly nonpolitical alliance. It pretty well had to be nonpolitical, in fact. Sharon Percy, 21, pretty, honey-haired daughter of Illinois' Republican Senator-elect Charles Percy, votes the way her daddy does. John D. Rockefeller IV, 29, lanky (6 ft. 6½ in.) nephew of the G.O.P. Governors of New York and Arkansas, votes the way no other Rockefeller does. He is a Democrat—and a fledgling politician who has just won election to West Virginia's House of Delegates. Still, when their engagement was announced last week, they looked like...