They knew it decades ago. They were just covering and have recently gotten their marching orders that Joe is O-U-T. If Trump is the nominee, however, you'll see them touting Joe as the Third Coming (after Jesus and Obama), the miracle that saves us all.
Even back in '87, I knew the man was as dumb as a box of rocks - just didn't think he suffered severe cognitive delay until 2008.
The news is abuzz with how the left is in full panic that they will lose their puppet - he's becoming so toxic to them, they're casting about for ANYTHING (they're pretty sure Gavin isn't ready to run, don't want Kennedy or Ms. "We need a department of LOVE" Williamson) - and even the press is doing it too. If they only realized - he's become toxic NOT just because his idiocy is becoming clearly evident - it's because he's actually been DOING WHAT THEY ASKED. Heck, even recently - he again said if he did or said this or that, he'd "get in trouble".
They're scared.
The rumor mill is, they will put the pressure on Michelle to run. I shudder at the thought - the woman has no leadership ability and anyone with an ounce of brains would know, she'd just be a stand-in for her husband. But I think people KNOW that, and would be willing to vote for her ANYWAY knowing it would really be a vote for Obama.