Please, NO Senator candidates (unless they have also spent considerable time recently in some executive position - preferably as a state governor). Also forget mayors.
Correct me if I am wrong here, was Richard Nixon, the last former senator to be elected president (I know he was a VP, but to have just held the office of senator at one time).
I don't think outside of Nixon, anyone after has ever held a senate seat and eventually gone to the white house.
They still have the title "Senator", though, and therefore, that is a problem.