I'll take a first crack, but I need you to pitch in and give me names, links, reasons.
Lieberman, Joe (V-CT), Senate
Feinstein, Diane (V-CA), Senate
Baucus, Max (V-MT), Senate
Rendell, Ed (V), Governor of Pennsylvania (Here's why.)
Ones I'm willing to think more about, but don't like much:
Casey, Bob (?), Wannabe Senator, PA
Nelson, Ben (?-NB), Senate