It would cost her more if she did not endorse him. It would mean she tried to get a man elected President who should not be in the Senate. That would not speak well of her judgment.
Plus it would expose her as a disloyal ingrate which would trouble me a lot more, when evaluating her as Presidential material, than a single senate seat.
..and don't think for a moment that all the media would be all over that one- not to mention Gibbs.