"It's fear of the unknown," said a senior U.S. national security official. "We don't know what he's really like under all the talk. . . . How will that play out over the next four years or even the next few months? I don't know if there is going to be a tidal wave of departures of people who were going to stay around to help Hillary's team but are now going to be, 'I'm out of here.' "