It is true that the State Democratic parties held on a little longer by separating themselves from the national party.
They held on for decades dunce; not a little longer. They also held onto the Congress of the United States.
Therefore, your argument and premise are patently false. Thank you for finally admitting the truth.
The claim that Republicans owned the black vote before the CRA is also an outright lie as is the notion that they left the Republican Party because the former racists on the Democratic side switched to the Republicans.