Condoleezza Rice (United States Secretary of State) 2

Successful politicians have a killer instinct, and there's no evidence of that here. She's also clearly indicated her disinterest in the job, and wants to return to academia. 

While her intelligence isn't in doubt, her performance at Sec State didn't remind anyone of Cordell Hull, Dean Acheson or Henry Kissinger, and the last thing McCain needs in electoral terms is another way to remind voters of the current incumbent.