They knew. Don’t think they didn’t know. They didn’t care, or more accurately, like the fact that he’s a usurper. They just misjudged; they thought everyone would continue to love him, and didn’t quite realize exactly how vile and nutso he is.
Recall that BO's "autobiography" Dreams From My Father was published in 1995, during the first Clinton administration.
I remember when the book came out (in the wake of the Rodney King arrest and Los Angeles riots, and the O.J. Simpson murder trial) and how heavily it was promoted. It seemed like some sort of novelty, this story of a "Kenyan boy makes good" story, something that was supposed to elicit a positive response in the reading public. The man on the cover seemed strange, with his fixed smile and his eyes as dark and unfeeling as a pair of black shoe buttons. I thumbed through the book but didn't think enough of it to buy a copy.
Someone was promoting him, way back when. See if you can remember back to the '80s and '90s when the foundations of his political career were being laid.
The 80s were not long after the Weather Underground fiends.