How did white Southerners erode the political right and earning power of African American in the late 1800s?

They passed laws restricting african Americans registration to vote.

Explanation:

The answer is very simple.

The southerners Democrats used their power in the state legislature to disenfranchise minorities, i. e the blacks, and ensure that Democratic Party and white power would not be threatened again by the blacks.

They passed laws restricting voter registration, and this stopped the reconstruction and prevented the blacks African Americans to earn power.