Robby Mook, 35
Robby Mook traces his history with Hillary Clinton back to the dark days of the 2008 Democratic primary, when the once-prohibitive favorite endured a slow-motion collapse across the map. His obsessive focus on organizing—both his own staff, down to 30-minute intervals, and the leave-no-voter-behind field program they assembled—produced a rare bright spot, helping her pull out an upset victory in the Nevada caucuses. The Columbia classics major’s no-drama style at the helm of that contest also yielded the “Mook Mafia,” a fiercely loyal corps of young operatives that now surround him as he captains Clinton’s second presidential bid. These days, his low-key resolve faces an even stiffer challenge as blowback from Clinton’s email controversy has once again knocked the frontrunner on her heels, raising questions about her campaign’s handling of the issue. How he adjusts for the recent events may be his greatest challenge yet.