Well, on the GOP side, McCain, Bush, and Rice are all tied in polling:
Rudy Giuliani 22
Condoleezza Rice 22
John McCain 21
Jeb Bush 11
Newt Gingrich 6
Bill Frist 3
None (vol.) 3
Unsure 11
On the Democratic side, I don't see who could actually beat Hillary:
Hillary Clinton 41
John Edwards 14
Al Gore 12
John Kerry 10
Joe Biden 5
Wesley Clark 4
Bill Richardson 3
Other (vol.) 1
None (vol.) 4
Unsure 6
http://www.pollingreport.com/2008.htm
If it's a choice between Hillary and some Bush clone, I don't see why Democrats would consider it a choice at all, regardless of how liberal they are. The only person I can see on that list who can beat her would be maybe Edwards - I think Kerry and Gore are done for just because they are branded as losers for not winning when they already had their shot.