Giuliani attacked Biden's remarks to a Virginia audience that Republicans wanted to roll back Wall Street regulations and put people back in "chains." Half of Biden's audience was black.
"I don't think he's nuts. ... I wonder if he's (Biden's) got the mental capacity to, you know, be the president of the United States" if something happens to President Obama, Giuliani told CBS News' "Face the Nation."
"I mean, this guy is like one gaffe after another, and he's a ...a joke on late night television," Giuliani said. "I think they've also locked him in his room for the rest of the campaign. I think the president scolded him, and, I mean, it's been one ... one strange incident after another, telling a man in a wheelchair to stand up. I don't know what's going on with him, but somebody should explain it."
Biden was defended by the No. 2 Democrat in the U.S. Senate, Sen. Richard Durbin of Illinois.
"Well, I can tell you, I served with Joe Biden for 14 years in the Senate, chairman of the Senate Judiciary Committee, chairman of the Senate Foreign Relations Committee. I've worked with him as vice president. I would just say, to paraphrase an earlier debate, I know Joe Biden. Joe Biden is my friend. And, Rudy Giuliani, that isn't fair. What you said has gone too far.
"And if you're going to put yourself out as the arbiter and the judge of vice presidential candidates, we're going to remember that, four years ago, you told America at the Republican Convention that Sarah Palin should be the vice president of the United States; she was ready to be commander in chief and president of the United States," Durbin said. "That speaks for itself."