WASHINGTON — The emails to Michelle Obama began flooding in minutes after she spoke out at an October rally in New Hampshire, her voice shaking, about Donald J. Trump’s treatment of women. Sexual assault victims recounted their trauma, fathers poured out anxieties about unhealthy influences on their sons, and a distraught parent agonized over how to explain rape to a 10-year-old.
The next morning in the East Wing, a first lady who had spent years in the White House staying away from politics sorted through a thick sheaf of printed messages — a selection of the 600 she had already received, an amount that would triple by the end of the day — and realized there was an unlikely finale for her.
Mrs. Obama had become the breakout voice of Campaign 2016.
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Dismissed early on by critics as an angry black woman unsuited for the tradition-bound role of first lady, she has emerged this fall as Hillary Clinton’s most popular surrogate, with soaring approval ratings that cut across party lines. Reluctant at first to engage in partisan politics, and conflicted when her husband decided to seek the presidency, Mrs. Obama has, almost in spite of herself, evolved into a powerful presence on the campaign trail.
“She has ended up to be the most effective and reassuring antidote to Trump that we have, and the best at making that contrast,” said Jennifer Palmieri, Mrs. Clinton’s communications director.
Celinda Lake, a Democratic pollster, said that Mrs. Obama had a special voice for women. “This isn’t a politician’s articulation of what’s going on here,” Ms. Lake said. “This isn’t a man’s articulation. It is a woman who never asked for this platform using it to say what she thinks.”
Mrs. Obama, 52, is no stranger to speaking her mind. A Princeton- and Harvard-educated lawyer who was forced to give up her career as a hospital executive with a $250,000-plus salary when Mr. Obama won the presidency, she nevertheless set about conforming to the traditional job of first lady. As the first black woman in the role, she was well aware from the start that there would be little margin for error.
She declared herself the “mom in chief.” She took on unassailable causes like healthy eating, exercise and military families. Despite the couture she wore to state dinners and the glamorous White House parties she threw with guests like Beyoncé, Mrs. Obama cultivated an unpretentious image by dispensing hugs rather than handshakes and kicking off her shoes to dance with local children on official trips abroad.
Inside the White House, the image is different: Mrs. Obama has come to be adored but feared in the East Wing as a tough and exacting boss who has little patience for mistakes, improvisation and wasted time. But her discipline and intensity have paid off in her success as first lady, staff members say.
The discipline extends to her daughters, who have been required to play a sport of their mother’s choosing in addition to one of their own and are barred from television or computer entertainment on school nights. Mrs. Obama has worked only two to three days a week to allow her to spend time with them and only participates in activities she regards as “value added.” She rarely grants an interview unless she thinks there is a compelling reason, such as a link to one of her initiatives. She declined to be interviewed for this article.
“We should do things that matter to us and matter to the country, but we don’t have to do everything,” Mrs. Obama told her staff shortly after her husband’s election.
That rarely included campaign politics. Until now.
“She’s gained confidence,” said Melissa Winter, her deputy chief of staff, who joined Mrs. Obama’s team in 2007. “I think she ultimately enjoys it more than she thought she might.”
When in 2003, State Senator Barack Obama of Illinois announced his campaign for the United States Senate at a news conference in a Chicago hotel, Mrs. Obama did not attend. When he weighed a run for the White House four years later, she resisted. Once she finally gave her consent, she was parsimonious in her time on the campaign trail.
Ms. Winter, who had just gone to work for Mrs. Obama, remembers it well. “When I first started, she said, ‘I’ll campaign two days a week, and I’m not going to work on the weekends, because I’ll be with the kids,’” Ms. Winter said. “And I’m thinking, ‘I moved to Chicago for this?’ Politics is not her thing.”
Her daughters, Malia, 17, and Sasha, 15, who were in elementary school when their father won the White House, were only part of the reason.
“Michelle once explained to me, ‘I try to organize my life not to have a lot of mess around, and politics is just a big mess,’” Mr. Obama said last month on Jimmy Kimmel’s late-night comedy show.
But this election, aides say, forced a decision by Mrs. Obama to wade into the muck. In September, while Mr. Trump was in a fight with a former Miss Universe he called “Miss Piggy” and an “eating machine,” Mrs. Obama sat down with her staff and began writing a campaign speech focused on Mr. Trump’s treatment of women.
“The more we talked about it, she felt that the debate had become so base that, when it’s time to cover your kids’ ears, it’s gone too far,” Ms. Winter said. “It was time to speak out.”
As Mrs. Obama was putting the finishing touches on the speech, an 11-year-old tape surfaced in which Mr. Trump boasted of grabbing women by their genitals without consent.
The result was a takedown of Mr. Trump that Mrs. Obama delivered in October in New Hampshire, where her voice trembled with emotion as she confided that the misogynistic tenor of the presidential race had “shaken me to my core.”
“Now is the time for all of us to stand up and say enough is enough,” she said. “This has got to stop. Right now.”
The following week, charged up in her new role, Mrs. Obama drew a crowd of 7,000 in Phoenix. “Here’s what I’m asking,” she said, imploring. “Don’t just tweet about my speech last week. If you liked that speech, then go vote. If you want to stand up for yourself and your fellow Americans, then go vote. If you want to get Hillary elected, vote. Vote early! Vote right now!”
Mr. Obama’s political advisers have long regarded his wife as a potent weapon. Their nickname for her in his 2008 campaign was “the closer.” Back then, with Mr. Obama engaged in a bitter Democratic primary against Mrs. Clinton, his aides noticed that Mrs. Obama’s so-called conversion rate — the ratio of voters who registered or signed up to volunteer or otherwise help the campaign after she made an appeal — was exceptionally high.
In part, that is because Mrs. Obama makes a point of telling voters what she expects them to do, like a mother ordering her children to do their homework.
“Don’t embarrass me, y’all,” she told voters in Charlotte, N.C., in early October, warning them that she demands the statistics after each campaign stop on how many people registered to vote or signed up to volunteer.
Still, Mrs. Obama knows all too well the darker side of putting herself out in the political arena.
In 2008, she was criticized as angry and unpatriotic after a comment at a rally in Wisconsin about the unexpected success of her husband’s campaign. “Let me tell you something,” she said, in remarks that haunted her throughout the campaign. “For the first time in my adult lifetime, I’m really proud of my country, and not just because Barack has done well, but because I think people are hungry for change.”
Conservatives who had been pushing racially tinged criticism of Mr. Obama, suggesting that he had been born in Kenya and portraying him as a radical liberal, turned their fire on his wife. A worried Mrs. Obama, who was furious about the racist rumors of her husband, told aides that she might be hurting her husband’s chances, and wondered whether she had been wrong to swallow her misgivings and plunge into the campaign.
“She wasn’t accustomed to this, and she wasn’t used to the criticism, and she became a lightning rod early on in the presidential race,” said David Axelrod, one of Mr. Obama’s closest advisers at the time. “It wasn’t like she was demanding to be out on the campaign trail. We asked her to be out there. And oftentimes, we threw her out there without adequate preparation or staffing.”
Mrs. Obama was livid when a New Yorker magazine cover cartoon in July 2008 pictured her wearing fatigues, combat boots and an Afro and toting a machine gun, fist-bumping with her husband, who was dressed in Middle Eastern garb and a head scarf. The couple was in the Oval Office, where an American flag burned in the fire place.
“Before she was well known on the national and international stage, she was caricatured by a few, and that was quite hurtful to her,” said Valerie Jarrett, a longtime confidant of the Obamas who serves as the president’s senior adviser.
The campaign hired Stephanie Cutter, an operative adept at handling political crises, to advise Mrs. Obama on her stump speech, her campaign itinerary and her media appearances.
The change was soon apparent. Mrs. Obama appeared on “The View” and talked about serving her daughters bacon for breakfast and hating to wear pantyhose. She gave an interview to Ladies’ Home Journal about her views on marriage and motherhood. She delivered a speech at the Democratic convention in Denver, where she talked of her working-class roots and called her husband “a great American story,” and — in a subtle attempt to answer her critics — spoke repeatedly of her pride in and love for America.
Her poll numbers shot up more than 10 points, above 50 percent, and by Inauguration Day, they were at 68 percent, according to Gallup’s daily tracking. They have not dipped below 60 percent since.
Mrs. Obama has often spoken of her dislike for living in the White House. “There are prisonlike elements,” she said in 2013 at a forum with Laura Bush. She added, however, that it was “a really nice prison.”
The fact is, Mrs. Obama has built a comfortable life there. Although family dinner in the residence is seen as sacrosanct, she has often been spotted making low-key escapes with a clique of female friends to the private back room of BLT Steak, an upscale eatery two blocks from the White House, or for martinis and Mediterranean tapas at the restaurant Zaytinya.
She has prowled the sidelines of Sasha’s soccer games, stayed up until the small hours at White House celebrity bashes and ventured to SoulCycle for private lessons with friends and young female aides, sweating in the dark with Secret Service agents on hand to protect her.
Over their time in the White House, Barack and Michelle Obama have adopted their own memes to explain each other to the public, each with a serrated edge that has the ring of truth.
Mr. Obama’s portrayal is tart: He casts his wife as the real boss in the family, one who has barely tolerated his political aspirations for far longer than she has wanted to, and is always on the brink of cutting him loose. (He told Mr. Kimmel last month that “Michelle would divorce me” if he were able to seek a third term.)
Hers is about a husband who is brilliant but takes himself too seriously and frequently needs an ego-shattering reality check. On “The Late Show With Stephen Colbert” in September, Mrs. Obama lowered her voice several octaves to impersonate the president lecturing his daughters on global warming at the dinner table. In 2007, she told Glamour magazine that her husband was “snore-y and stinky” when he awoke in the morning.
Cindi Leive, the editor in chief of Glamour who has worked with the first lady on the “Let Girls Learn” education initiative, recalled editing that first interview and thinking she wanted to know her better.
“That’s exactly the sentiment that much of America has come to have over the last eight years: ‘I need to be friends with this woman,’’’ Ms. Leive said. “‘Why am I not friends with this woman?’ And, then increasingly, ‘I am friends with this woman.’”
Christine Matthews, a Republican pollster who has specialized in studying gender, said Mrs. Obama, by confining herself to popular initiatives as first lady, has built a reservoir of trust and popularity that puts her on the level of a Barbara Bush or Laura Bush, rather than Nancy Reagan or Mrs. Clinton, who were seen as more divisive.
“She could have gone in a really bad direction but didn’t, and she has really evolved,” Ms. Matthews said. “Unfortunately, there are gender lenses when you look at women who have political ambition. The American public doesn’t want that in a first lady. They want someone nice and relatable.”
But as voters greet Mrs. Obama rapturously at campaign events this fall and eagerly chant her slogan, “When they go low, we go high,” speculation has started that she may one day seek office herself. Those close to her regard the prospect as absurd.
“I am certain she does not wish to be in politics herself,” Ms. Jarrett said. “There are very few things I’m as certain of in life.”
One other thing is certain: Republicans, who thought nothing of attacking Mrs. Obama in 2008, now shy away from doing so, a testament to her popularity and appeal.
“We don’t go after the first lady,” Sean Spicer, the communications director for the Republican National Committee, said in July. “Full stop.”