What is a likely explanation for the fact that southern african americans were elected to congress during reconstruction but no norther african americans were elected until 1928

The most likely explanation is that towns, states, and voting districts in the South had much higher numbers of African American voters whereas there were far fewer African Americans in the North. Remember that while African Americans were elected almost immediately in the South, Jim Crow and voting laws instituted soon thereafter boxed them out completely (because white Southerners knew the number of African American voters would vastly outnumber white voters in many areas). 

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