ROCK HILL, S.C. — A generational divide among black voters is persisting in the Democratic primary — between the two old white men. Joe Biden has amassed a staggering lead among older African Americans, commanding nearly two-thirds support of black voters 65 and older in the most recent Morning Consult poll. Bernie Sanders is the favorite of black millennials, though his margin with that group is much smaller. Among all black voters, Biden is leading Sanders, 41 percent to 20 percent. Biden and Sanders have maintained their edges even as other candidates — including two African American senators, Kamala Harris...