Joe Klein writes:

Biden on McCain: Biden's opinion of his old friend John McCain has changed significantly--for the worse--during the course of this campaign. Biden called me in June to express his amazement that McCain continued to insist that Mahmoud Ahmadinejad was the leader of Iran, even after I pointed out--during a press conference--that the Supreme Leader Ali Khamenei controlled Iran's foreign policy and nuclear program. McCain's response was that the "average American" thought Ahmadinejad was Iran's leader...and Biden proceeded to jump all over that in a subsequent interview with Think Progress:

I don’t want an average American as president.... [A]verage Americans don’t want an average American president of the United States of America. I want someone above average. I want someone who knows what they’re dealing with. And it surprises me that John didn’t understand the complexities of the power struggle going on in Iran right now.

Biden told me that he was amazed and disappointed by the changes in McCain during the course of the campaign. "I just don't recognize the guy anymore," he said. "It's a shame." No doubt, Biden will have more to say on that subject now--which is one of the great advantages of picking him for vice president: according to the polls, foreign policy is McCain's greatest strength. In reality, McCain has been captured by neoconservative extremists and is proposing an extremely dangerous course of action internationally. Biden has the stature and knowledge--and the blue-collar, no bull pugnacity--to call McCain on his imprudent militarism.

Another advantage: If McCain does go with Romney, Biden would have a significant knowledge advantage in the vice presidential debate. (And if McCain goes with Joe Lieberman, I'd be willing to pay good money to watch that one.)