Survey USA's state by state polls showing how Obama and McCain would fare against each other with various permutations of running mates continue to show that John Edwards is by far the best possible running mate for Obama among the possibilities they are testing (Ed Rendell, Kathleen Sebelius, Chuck Hagel, Edwards.)
This polling might make you think Edwards should be Obama's running mate, but consider some historical perspective:
In late May 2004, a Mason Dixon poll showed George W. Bush leading John Kerry 48-41 in North Carolina. It also showed that with Edwards on the ticket, the margin narrowed to 46-45.
Of course Edwards was chosen- and that seven point deficit turned into a 12 point loss.
The lesson? The Vice Presidential selection is certainly important, because it will line someone up as the leading contender to be our next President- but the running mate is very unlikely to have much of an impact on who actually wins this fall.