And the Klan moved north into states like Indiana and ran them in the '20s. Post WWI the Republicans abandoned African Americans completely and the Democrats were decades away from cultivating their northern city branches of settlement; during the Depression they were last in line too.
I have the impression, perhaps quite wrong, that part of what Huey Long did with his "Share the Wealth/Every Man A King" demagoguery was to help African Americans via backdoor routes even though they did not actually vote; he would not attack white supremacy head on but he did subvert it in various ways. Whether that was the act of a would-be dictator or just a man who looked at the structure of discrimination and as a genuine grassroots populist it revolted him, I do not know--indeed he may have been a hardliner racist though my impression remains opposite. It helps me to hope that the mere sight of injustice does motivate people to move out of their comfort zones and take some risks to alleviate it. Harry Truman also, despite being raised in a very pro-Dixie household, claimed to be motivated to end discrimination out of personal outrage at the humiliations he witnessed, which did strike him harder when he realized some of these victims of racist terrorism were WWI veterans. His own Army service was not in the trenches but it was in harm's way as an artillery officer and he did have sympathy for veterans in general--and poor folk, in general.
But in terms of formal patronage of the established parties and even the larger of the revolutionary ones, African Americans were abandoned in the 1910s-30s. Toward the later years, the rise of urban ghettos in the north began to give them some political clout, and surely Truman and Long understood that too.