It's not just low. It's nonexistant. Look, pretend you are a Democratic voter, and you'd be perfectly happy with Clinton or Obama.
Ask yourself this. "What will I and all of my Democrat friends say the day after McCain wins?"
(Aside from, "No fair, I think they only counted three of my votes.")
They're going to say, "Damn. We should have gone with Hillary."
She's doing the exact right thing. She ran a good race, she was magnanimous at the convention, now she's fading away into the background to nurse her wounds. When the Democratic party needs a champion, she'll be back. And the Democratic voters will say, "We should have gone with Hillary LAST time."
Too easy. Why muddle things up by jumping on the ticket now, just as that ticket is taking a mach 5 nosedive into the pavement?