If blacks are owed reparations, the Democrats are solely responsible for paying them.
Determining who your ancestors voted for before the Civil War (if they were living in the US) would be harder in most cases than proving that you had an ancestor who was a slave in the US before 1865. Of course African Americans won't be asked to provide a family tree--they will be presumed to be descended from American slaves (although some may be descended from Caribbean slaves).