It's the electability, stupid should be one of the major themes of the Obama campaign if they want to play harball with Hillary. The Democrats are desperate to take back the White House and the polls suggest that he is far better placed to do that than her.
Just look at the numbers when you pit the two of them against the current Republican frontrunner Rudy Giuliani:
Giuliani 50Clinton 44
Obama 52Giuliani 43