A dazzling sunset is falling over the New York harbor as thousands of people crowd into the Brooklyn Navy Yard’s Duggal Greenhouse in early June for the culminating evening of Hillary Clinton’s long primary run. The floor is dense with supporters. Music is playing—“Ain’t No Mountain High Enough,” “I’m Every Woman”—as MSNBC’s election returns stream onto an enormous screen; whenever a new state is called in Clinton’s favor, the entire greenhouse bursts into cheers. At a quarter to ten, the news crawl announces, Clinton to speak about historic win, and the network cuts to the floor itself. “Hil-la-ry! Hil-la-ry!” the crowd chants, seeing itself on-screen. The lights dim, and a mini-documentary begins to play. When it ends, the evening truly begins.

“Ladies and gentlemen, please welcome our next president of the United States, Hillary Clinton!”

The audience churns as the Clinton family weaves toward a platform in the center of the hall. The candidate is dressed in striking white. “Thanks to you, we’ve reached a milestone,” she says, shouting above the cheers. “The first time in our nation’s history that a woman will be a major party’s nominee.”

The call is heard around the country, but for Huma Abedin—Clinton’s assistant, adviser, and professional confidante for two decades, who is watching backstage—it has even deeper import. “You always try to keep a sense of the history in your mind, and it creeps up fleetingly as you go through the motions of your day-to-day,” she says, “but it overcame us that night, really at that moment.” Just before Clinton walked onstage, Abedin whispered in her ear: “Take a deep breath, look around the room, and savor the moment.”

Powerful, glamorous, and ubiquitous, Abedin is in many ways the engine at the center of Clinton’s well-run machine, crucial and yet largely out of sight. She presides over the candidate’s public appearances, helping set message points and optics. She coordinates much of the campaign’s work around the office, bringing a large, sprawling operation in line with Clinton’s vision and voice. And she is a key point person for the most essential parts of the coming Democratic convention: who will speak, what they’ll say, and in what order they’ll appear.

To onlookers, Clinton and Abedin seem to travel the world as a single entity joined by complementary strengths. If Clinton spends her life at the microphone, Abedin is constitutionally circumspect. If Clinton, in her bold suits and impeccable coifs, distills a certain era of feminist empowerment, Abedin, with her breezy downtown dresses and mobile power-dialing, is the professional face of a younger, more wired-in female generation. As Clinton’s longest-serving staffer, she is both the campaign’s deepest memory and its farthest-seeing eye—a woman who, more than anyone besides the Clintons themselves, can envisage the sort of president that Hillary will be, based on her work in the White House, the Senate, and the State Department, and who’s down with what she sees.