According to Aaron David Miller, who worked for six different Secretaries, success at Foggy Bottom depends on a close relationship with the President:
The president must have your back. Without this, the game is over before it begins. All presidents support their secretaries of State, but not all get the support critical to success. Baker used to say that he was President George H.W. Bush’s man at the State Department, not the State Department’s man at the White House. Those two were particularly close, and it gave Baker real authority, power and street credibility.
And for that reason, Miller thinks Ambassador Rice is the strongest candidate to succeed Secretary Clinton.