What about Paul Cellucci? His is the name I've heard mentioned most-often about a possible Senate run on the GOP side.
That's a good point. I honestly think that it would take a Kennedy retirement to make the race competitive for Card, Romney or Cellucci. This is the only state in the country where the Kennedys are still thought of as royalty, but I'm not sure that it extends to his nephew Joe.
Even for an open seat, it will be an uphill struggle for any Republican. All of the Rats in the congressional delegation (Frank, Markey, Meehan, Delahunt) have been able to raise money hand over fist each election cycle while facing token opposition.