Hillary is probably unelectable in a national election. Those
elections are decided by swing voters, and with Hillary, you either
love her or hate her. She's not going to do well with swing voters. If
the contest is between her and McCain, I think you'll see a lot of
moderate Dems voting for McCain, because there is no way they will
vote for Hillary if they have an escape route that is at all
palatable.
Your guess is as good as mine on this, but I think a Hillary-Barack
ticket would be unstoppable.