In the late nineteenth century, southern Democrats redrew the boundaries of congressional districts to .

reduce the power of the African-American vote

Explanation:

In the late nineteenth century, southern Democrats redrew the boundaries of congressional districts to "reduce the power of the African-American vote."

This process is often referred to as Gerrymandering. It is one of the many means by which the Southern Democrats used to disenfranchise or reduce the power of the African-American vote.

Other methods used were poll taxes, literacy tests, grandfather clause, etc.

it increase by 12% a year from 1812-1820