WASHINGTON — As the approval ratings for presidential candidates go lower, the first lady's go high.

Michelle Obama, the reluctant campaigner who can't wait to move out of the White House, has suddenly become Hillary Clinton's secret weapon, capturing headlines with passionate and personal speeches promoting the former secretary of State and castigating her opponent.

White House aides say the first lady is more valuable on the campaign trail than even the president — "probably the most powerful advocate that Secretary Clinton has," said press secretary Josh Earnest.

Just as President Obama's effort to get Clinton elected has made him the most active lame-duck presidential campaigner in history, historians say Michelle Obama is perhaps the most active first lady to campaign for someone other than her husband.

Before Clinton — who is, after all, a former first lady and senator herself — Eleanor Roosevelt was the most active first lady in history. Sen. John F. Kennedy repeatedly courted her for her support in 1960, and when it came it was more in the form of behind-the-scenes advice than public pronouncements.

Michelle Obama, on the other hand, has taken an ever more visible role on the campaign trail, with recent speeches in New Hampshire and Arizona that garnered national television audiences. She'll appear side-by-side with Clinton on Thursday in Winston-Salem, N.C.

"I don’t think we’ve seen anything like this before," said Kate Andersen Brower, the author of First Women: the Grace and Power of America's Modern First Ladies. "She is a godsend to the Clinton campaign."

That's partly because Mrs. Obama complements Clinton's perceived weaknesses. "Michelle is kind of a moral compass," Brower said. "She can talk about topics that Hillary Clinton can’t. Clinton hasn’t really talked about her husband's transgressions in the White House. The Obamas don’t have that same stain on them."

That's partly because Michelle Obama has largely been above the fray of partisan politics during the past eight years, choosing to focus on relatively non-controversial causes like nutrition, military families and the rights of young girls around the globe.

Brower points to what has become Michelle Obama's signature line of the 2016 campaign. "When they go low, we go high,"

"She can say that," Brower said.

At a Democratic fundraiser in California this week, President Obama said the first lady would just as soon stay off the campaign trail,

"Some of you may be aware of the fact that Michelle does not really love politics. This was not her first choice for me. She would have preferred a quieter life, a little bit more out of the limelight," he said. "But the passion that she's brought to campaigning this time speaks to the degree that this election is different, the choice is different.

"So part of the reason Michelle is working the way she is, is because she understands, as I understand, that some more fundamental values are at stake in this election," he said. "It has to do with our basic standards of decency, how do we treat people."

Perhaps more than anyone else in the 2016 campaign, Michelle Obama has framed the choice between Clinton and Republican presidential nominee Donald Trump as a question of character. "She’s giving speeches that are very sort of value-laden and personal and to her," said David Axelrod, Obama's former campaign strategist, to conservative talk show host Hugh Hewitt last week.

In a speech in New Hampshire this month — the week after the leaked Access Hollywood tape revealed Trump describing sexual assaults on women — she provided a scathing indictment of Trump's behavior tailored to the day's headlines.

"Here I am, out on the campaign trail in an election where we have consistently been hearing hurtful, hateful language about women," she said. "And I have to tell you that I can’t stop thinking about this. It has shaken me to my core in a way that I couldn’t have predicted. So while I’d love nothing more than to pretend like this isn’t happening, and to come out here and do my normal campaign speech, it would be dishonest and disingenuous to me to just move on to the next thing like this was all just a bad dream."

Such is her popularity that even Trump himself — famous for late-night Twitter broadsides against Gold Star families, Miss Universe contestants and television anchors — has said barely a word about the first lady, even as she attacks him.

"We have a president, all he wants to do is campaign. His wife, all she wants to do is campaign," Trump complained last week.

Michelle Obama's approval ratings are head-and-shoulders above Clinton's, Trump's or even President Obama's. An August Gallup poll put Michelle Obama's approval ratings at 64%. And they're higher than those of the two possible candidates to succeed her in the East Wing: Bill Clinton (49%) and Melania Trump (38%).

"She is somebody who enjoys the deep respect of a large majority of Americans," Earnest said. "She also is somebody who's a very persuasive speaker, she is somebody who has been able to make a forceful personal case about why she's involved in this election. And yes, I think the president would admit that his wife is an enormously influential and powerful surrogate in support of Secretary Clinton."

Previous first ladies, like Barbara and Laura Bush, maintained high popularity by largely staying above the political fray. Michelle Obama's approval ratings actually increased by 6 percentage points after her Democratic National Convention speech.

In that speech, she spoke directly about issues of gender and race, talking about watching her daughters grow up in a White House built by slaves. And she prosecuted the character argument against Trump in a way that only the mother in chief could. "With every word we utter, with every action we take, we know our kids are watching us," she said. "This election, and every election, is about who will have the power to shape our children for the next four or eight years of their lives."

Would Michelle Obama convert her campaigning skills into political office of her own someday?

Asked that question last week, the usually verbose Earnest gave a one-word answer: "No."