The U.S. government is not the culprit and bears no liability in this case.
There was slavery longer under the US flag than under the Confederate flag.
The democrat party, however...
“The U.S. government is not the culprit and bears no liability in this case.”
It was always illegal to enslave someone in the United States, from its founding.
After the USA came into existence, new slaves were either born to existing slaves, or purchased from outside the country. 20 years after the Constitution of the United States went into effect, slaves could no longer be legally imported. So no one was ever enslaved (had their previous liberty taken from them) under the auspices of the USA.
Also, one alive in the United States would have standing to sue.
New Deal and Great Society social welfare programs, and public education, constitute almost a century of reparation for any residual economic effects.