African Americans in the deep south prior to the late 60s were not democrats. It would be like joining the Klan.
Yeah, agreed. Probably the closest to a black democrat at the time was WEB Du Bois, and that guy is a Communist and closer to a member of the Socialist party than a Democrat. Now, WEB Du Bois, unlike MLK, actually IS confirmed to be a Soviet agent, openly rooting for the Soviets every single dang time.
Those who were able to vote generally were not, but they were largely disenfranchised (otherwise districts in MS & SC would’ve been sending Black Republicans to office since Reconstruction). Although my state of TN was not “Deep South”, many Blacks in Memphis/Shelby County were permitted to vote by the notorious Democrat Boss Crump in order to control state/federal politics. Blacks would be directed to vote for the Democrat candidate of his choice, when and if needed.
Add to that, for many left-leaning activist Blacks in the FDR era onwards, they were generally more supportive of the National Democrat party, even in the South. Many of their Northern counterparts who could vote had jettisoned the GOP by 1936 and were majority (but not overwhelmingly) Dem. It took barely a dozen or so years before most were supporting the Dems by 2/3rds margin (with some deviations based on particular contests) and could cease to be counted on by Republicans en bloc. By 1964, it was uniformally 90% and has remained stuck at that % since.