Robert Byrd was a good Democrat, so that overrides his Klan membership, in the view of the liberals.
I understand what you are trying to say, but Byrd ended his association with the Klan in the 40s. But being a democrat, he was excused. If he was a Republican, he would never live it down. However, Trump had dinner with a racist in the last week. Kind of different. Trump put himself in harm’s way, so let him fix it.