And what it means to Mrs. Obama is sacrificing many of the things she holds most dear, in favor of a larger goal. Although she has concluded that this mission is worth what it takes, achieving such acceptance has been difficult, and the adjustments are ongoing.

Her husband’s vaulting ambition was always a given. “She knew what she was getting into,” says Craig Robinson, Michelle’s brother, who is the men’s basketball head coach at Brown University. “Ever since I’ve known Barack, he’s said that he wanted to be in politics. He’s never changed his interests; it just happened that he’s turned out to be very good at it.”

But Obama’s accelerated timetable came as a shock, even to his spouse. As recently as 2000, Obama—then a little-known Illinois state senator—lost his first race for Congress. But in 2004 he ran for the U.S. Senate and won in a landslide victory, collecting more votes than any Illinois politician in history and becoming the nation’s only black senator.

No sooner had his family adjusted to that development than he started to consider a run for the presidency in 2008. “This was a sudden decision,” Mrs. Obama says. “He had just won his U.S. Senate seat. In my mind, it was ‘O.K., here we are—you’re a U.S. senator.’ ”

With Barack commuting to Washington while Michelle and the children remained in Chicago, the family’s life was already complicated, and Michelle was stunned when Barack abruptly fast-forwarded his presidential aspirations. “I thought, Uhhhh—you’re kidding! It was like, No, not right now—right? There was a period of ‘Let’s not do this now; let’s press the “easy” button! Can we get a break, please?’ ”

She sighs and rolls her eyes. “So we had to talk about it. Before I signed on, I had to know, in my mind and my heart, how is this going to work for me, and would I be O.K. with that? He wouldn’t have done this if he didn’t feel confident that I felt good about it, because it is a huge sacrifice. The pressure and stress on the family isn’t new. But we entered this thing knowing it was going to be really, really hard. For us, the question was: are we ready to do something really hard again, right after doing something that was really hard?”

Barack eventually managed to persuade the whole family that his quest justified the cost. “This is something that’s bigger than everybody,” says Craig Robinson. “This is so important that it’s worth her saying, ‘I’ve got to rethink the way my family is structured.’ ”

Although Mrs. Obama had recently been appointed as vice president at the University of Chicago Medical Center, her husband’s announcement in February 2007 that he was entering the presidential race forced her to scale back her professional commitments. In May, she reduced her work schedule to 20 percent in order to meet the needs of her family while participating in the campaign.

“She was intending to take a full leave of absence, but she couldn’t make herself separate from this professional career that’s been so important to her,” says Susan Sher, vice president for legal and governmental affairs at the University of Chicago Medical Center, where she is Mrs. Obama’s boss. “Her involvement in her work life has been so serious that it’s not easy to just say, ‘Never mind.’ ”

Mrs. Obama has a long history of speaking out about the ways in which men’s choices—particularly their professional ambitions—often leave their wives to pick up the slack, even when they have their own careers. “What I notice about men, all men, is that their order is me, my family, God is in there somewhere, but me is first,” she told the Chicago Tribune in 2004. “And for women, me is fourth, and that’s not healthy.”