Jake Sullivan, 39
The Minnesota native earned Hillary Clinton’s trust as an adviser to her 2008 presidential campaign, and at the State Department, where he was the youngest-ever director of policy planning. After she left, Sullivan stayed on to see through the Iranian nuclear deal he was instrumental in launching. He rejoined Clinton for her second run, expanding his foreign-policy portfolio to include domestic affairs and outreach to businesses. If Clinton wins, look for Sullivan to take a senior White House post, possibly deputy chief of staff.