That would effectively be the end of the election for Obama. He can’t give people the impression that he couldn’t he even get his decision on a running mate in the correct column. It would be too late, and the hope of getting a new pick out into the public eye would be massively shrouded by the story of Biden getting dropped.
Put it this way, if he dumps Biden, he’s telling the country, “I think I’m going to lose”, and, frankly, people don’t vote for losers.
Good synopsis.