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After the Civil War, African-Americans typically voted Republican. However, the Great Depression and New Deal led the black electorate (which was largely composed of urban Northern voters as most Southerners were disenfranchised) to vote overwhelming for FDR in 1936. This trend continued over the next few elections and was solidified after the passage of the Civil Rights Act of 1964 and the Republicans' nomination of Barry Goldwater.
With a PoD after 1936 (though, a later one is always more interesting), have the African-Americans be considered either a safe Republican or Republican-leaning demographic.