If Clinton starts to attack Obama it will be the end of her not him, especially if he refuses to stoop to that level. She's already seen as a ruthless, vindictive person and she cant afford to lose any other voters in both the primary and general elections.
Its a shame that the best canidate in the party isnt getting the face time he deserves. Bill Richardson is probably the most qualified and has the most experience than everyone but Biden (Who needs to drop out asap). If Richardson cant get the nomination, and I doubt he will, I hope that he at least gets the chance to be VP.