Romney endorsed O’Donnell in her primary run against Castle.
If that's true, and I have no reason to doubt you, it seems odd that Rove immediately came out on FOX and went on a rant of how COD was too unqualified, inadequate, and inexperienced to represent Delaware. Especially since Romney was clearly in the good graces of Rove at that time (and apparently still is).
In addition, my understanding was that an O'Donnell ‘no’ vote on the impending obamacare legislation could have stopped it from becoming law, or at least put a significant wrench in the works. The Delaware senate seat was that important.