Actually, the issue of slavery is most likely a lesser problem with the left than capitalism and the current constitution.
Turning the U.S. into The Socialist States of America is their ultimate goal. If no slaves had ever been imported from Africa, they'd still be trying to tear down the U.S.
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Goat Prairie, a reflection birthing a suggestion.
If the objective is to change hearts and minds, suggest slavery is a far
more powerful subject, as its opposition is derived from spiritual values.
If instead you chose capitalism, its contra is socialism; both being
economic isms that are grounded in materialism.
As the Spiritual (the Almighty) created the Material (Nature, the Universe);
intuitively the former is the superior of the latter.