According to the head of his vice presidential vetting committee, Sen. John McCain wanted to choose Sen. Joe Lieberman as his running mate last year but was advised against it because West Virginia law would not allow the selection of a non-Republican on the ballot in that key state.
However, ballot access expert Richard Winger says McCain received bad legal advice and could have picked Lieberman after all.
Update: Rick Hasen has his doubts that it was bad legal advice.