PHILADELPHIA—Donald Trump has energized the plentiful working-class, white voters in Pennsylvania, but Hillary Clinton’s lead among African-Americans here and her growing hold on suburban voters have suddenly made Republican hopes of winning this battleground look more remote.

Mrs. Clinton’s lead in the RealClearPolitics aggregate of polls had narrowed to less than a single percentage point in June, but it has now grown to nearly 10 points. A top super PAC backing Mrs. Clinton, Priorities USA, on Monday said it was suspending...