How about this: slavery was bad. The Democrats were the party if slaveholders while the Republican party was founded by abolitionists. Joe Biden is a Democrat and Donald Trump is a Republican. End of story.
At 30 years per generation, it would be the great great great grandparents of the black children in schools who were born slaves but freed by the Union Army. Add one more "great" if you average 25 years per generation. Does anyone know about their great (x 3) grandparents? I remember two of my great grandmothers who died my youth. Other than that all I know is some old county property maps which have my mother's family name on one patch of a couple hundred acres in the late 1800s. How long ago does something have to be to say "that is history, but it does not control who you are and who you will be"?
Well...
If you go by a century, 100 years divided evenly by 4 equals 25, or 4 generations. (Divided unevenly by 3rds yields 3.3 generations.) Using 25, there have been 6.3 generations since 1863 when the Emancipation Proclamation was signed.
A pedigree is usually plotted out to the 5th generation, with the first generation being the two parents. By that method, there are 64 great-great-great-great grandparents in the 6th generation. A person shares only 1.56% DNA with their 6th generation antecedents (depending on calculation method). Were all 64 of those people slaves? How many and which ones?
There obviously are no Americans alive today whose parents were slaves in the pre-1863 US, but it's not impossible for someone alive today to have had grandparents who were slaves. The odds are very, very small though. Average life expectancy in the US approximately doubled since 1863, but not steadily. Also, there are some examples of men who married a much younger woman late in life and had children by that younger wife. Then those children also had children late in life. An example of this is a grandson of President John Tyler who is still alive at the age of 92, but the story of the Tyler family is unusual.
Bottom line: the overwhelming majority of people alive today who descend from US slaves would be a 5th or 6th generation, or more, descendant -- meaning they have only negligible remaining DNA relationship to those slaves.