"I was scared because I had never seen Joe like that before or since. Never," she says in an interview with CBS News.
The then-president's performance in the key debate during the re-election campaign was later labeled a disaster - full of babble and misstatements.
"I don't know what happened. When I looked, I thought, 'Oh my God, he's had a stroke.' It scared the hell out of me," says Jill Biden.
After the debate, there were loud calls for Joe Biden to suspend his campaign. A month later, he withdrew his bid for a second term and instead supported his vice president, Kamala Harris, as the Democratic nominee.
The full interview will air on the CBS program Sunday Morning on Sunday.
