It's been an unfortunate truth that the US has long been a country that's flirted with fascism. Ultimately, Thaddeus Stevens was right in his conviction that after the civil war the southern states should've been completely crushed and the land given to the freedmen.

The Constitution was clearly written for rich land owning white men first of thought, and everything else being left out or only in fractions. They added some checks and balances as a hand wavy idea of trying to stay away from autocracy, but they kind of made them toothless. I'd guess they just didn't have the imagination that people would willingly allow someone to go back towards autocracy since they were fighting so hard to leave it.