McCain being more of a "Foreign Affairs" type president, and crossing the isle too many times, continues to anger the conservative wing of the republican party.
While Romney is not perfect, he is a fomer business man that I think could help McCain with his economic policies (McCain's weak point). He would balance the ticket a lil more as Romney is more of a moderate/conservative then McCain is. Although people are still old fashened about religious beliefs (Romney-Mormon). I think that would be another reason Romney may get over looked for a VP nod.
