"Today I love my husband more than I did four years ago, even more than I did 23 years ago when we first met," she said to ecstatic roars from the crowd in Charlotte, N.C.
"I love that he has never forgotten how he started. I love that we can trust Barack to do what he says he's going to do, even when it's hard — especially when it's hard."
Obama campaign aides said earlier Tuesday that the first lady would serve as a "character witness" to President Barack Obama's years in the Oval Office when she took to the stage in Charlotte.
She went far beyond that, delivering a powerful address that not only delved into the couple's past — their shared values, upbringing and financial struggles — but also highlighted a list of Obama's biggest legislative accomplishments, in particular so-called Obamacare.
"When it comes to the health of our families, Barack refused to listen to all those folks who told him to leave health reform for another day, another president," she said.
"He didn't care whether it was the easy thing to do politically. That's not how he was raised. He cared that it was the right thing to do."
As she recounted her husband's early career, the first lady also took what appeared to be some veiled but well-aimed shots at Mitt Romney, Obama's wealthy Republican rival for the White House who spent much of his career at the helm of a private equity firm.
"(Obama) is the same man who started his career by turning down high paying jobs and instead working in struggling neighbourhoods where a steel plant had shut down, fighting to rebuild those communities and get folks back to work," she said.
"Because for Barack, success isn't about how much money you make, it's about the difference you make in people's lives."
There was another apparent jab as she recounted how she and her husband were raised by their parents and grandparents.
"We learned about dignity and decency — that how hard you work matters more than how much you make, that helping others means more than just getting ahead yourself," she said.
"We learned about honesty and integrity — that the truth matters, that you don't take shortcuts or play by your own set of rules, and success doesn't count unless you earn it fair and square."
The crowd cheered at the mention of the word "truth." Romney's running mate, Paul Ryan, has been accused of larding his own speech with falsehoods and exaggerations at the Republican convention last week in Tampa.
The speech was a triumph for Michelle Obama, who has admittedly been a very reluctant political spouse throughout her husband's career. She said as much again on Tuesday night, saying she feared the impact on her children once they moved to the White House.
The last time she spoke at the party's convention — in 2008 — she was under fire, assailed by conservatives as unpatriotic for remarks she'd made months earlier about her husband's political fortunes.
"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," she said during primary season in comments that immediately set off a right-wing firestorm.
It's been a transformative uphill climb ever since, said Myra Gutin, a first lady expert and communications professor at Rider University in New Jersey.
"She had a lot to prove after that comment, but since then, she's really been at the top of her game," Gutin said in an interview.
"She's pretty much been a flawless first lady. She selected a cause that gets a lot of response, it resonates with people; there aren't very many people out there who would disagree with her message that American kids need to get healthier."
Another pet project, assisting military families, has also been a home run for the first lady, she added.
Indeed, after an almost entirely gaffe-free four years in the White House, the Chicago-born lawyer's popularity now far surpasses that of her husband. When Gallup last pitted the couple against one another in May, Michelle Obama had a 66 per cent approval rating while her husband was at 52 per cent.
The president watched his wife's speech from the White House, cuddled with the couple's two girls. Earlier in the day, he predicted he'd get "all misty" as he took it in.
"Whatever I say here today, it's going to be at best a distant second to the speech you will hear tonight from the star of the Obama family," he said as he campaigned in Virginia. "This is just like a relay, and you start off with the fastest person."
With her folksy sense of humour and willingness to poke affectionate fun at her husband, Michelle Obama has also proven to be one of the president's most effective weapons in his battle to win re-election.
She's not shy about drawing a contrast between him and Romney, even though she rarely mentions the Republican by name. Romney, she suggests, was born with a silver spoon in his mouth and has no clue about how most Americans live and support themselves.
Barack Obama, meantime, has shown what he's made of since becoming president, she said on Tuesday.
"Today, after so many struggles and triumphs and moments that have tested my husband in ways I never could have imagined, I have seen first-hand that being president doesn't change who you are — it reveals who you are," she said to roars of approval.
"So when people ask me whether being in the White House has changed my husband, I can honestly say that when it comes to his character, and his convictions, and his heart, Barack Obama is still the same man I fell in love with all those years ago."
Romney and Obama have been running neck and neck in most public opinion polls with the presidential election three months away.
Like Romney, Obama is hoping for a "bounce" in the polls following the convention that may put some wind in his sails heading toward Nov. 6. Romney's bounce, however, represented only a couple of percentage points after last week's Republican convention in Tampa, Fla.