The prospect of African Americans voting in large numbers was troubling to

The answer is whites in the South.

The correct answer is - the Republicans.

The prospect of the African American population voting in large numbers was a big problem for the Republican political party. This was due to the fondness that the African American had for the Democrats and were going to vote for them, and thus significantly affect the final outcome of any election. This is still the case nowadays, where we have situation where over 95% of the African Americans vote for the Democrats, even though they haven't seen pretty much anything done by this party for their interest, they continue to prefer them on the elections, something like a traditional voting.