They know they have no "why you should vote for us" message so they have nothing but pathetic false attacks to rely on.
Even if he did say that, it was so obviously a joke. The joke wouldn’t have been funny if he didn’t think the KKK was evil. Same as Trump and the p-word remark: if the word hadn’t been extreme, it wouldn’t have served the purpose of the statement.
Political people on our side have to learn that you can’t make jokes. One mention of a loaded word, and the Left takes the whole thing the wrong way, intentionally, so they can make political hay out of it with stupid people, or else because they are just plain stupid and don’t understand humor.