Romney, McCain, and Huckabee have ALL said CRAP against each other yet Romney and Huckabee are now both campaigning for McCain.
I think that Obama needs to campaign by himself for awhile and perhaps Hillary could campaign for him in the Fall.
Primaries produce all sorts of nastiness. It will be up to Hillary Clinton to undo her own pro-McCain remarks.
At the end of the day, the positives of Hillary Clinton rallying her own supporters to Obama faf outweigh the risk of pairing her own past comments with current sentiment.