The U.S. constitution counted slaves at 60% of a person. Some of the arguments for the three fifths compromise were that slaveholders didn't want people who weren't "knowledgable" about slaveholding to be meddling in slaveholder's business. This is essentially one of the causes that sparked the Civil War.
I don't understand why Americans are so against becoming a full democracy. It's not like the current system benefits anyone. Congress, presidents and our courts are dysfunctional. The government is becoming the laughingstock of the world. At least in a democracy there's more chances of unified rule instead of 6 years of gridlock followed by 2-years of whiplash changes.