Michelle Obama and Democratic presidential candidate Hillary Clinton reportedly don't have the most congenial relationship, owing to tensions that can be traced back to the bitter 2008 primary campaign.

But now, eight years later, as Clinton makes her run for the White House, the president's wife may be Clinton's most effective political advocate.

"I think she's a tremendous asset," said Myra Gutin, a communications professor at Rider University and author of The President's Partner: The First Lady in the Twentieth Century. "She is just a tremendously effective surrogate. Michelle Obama is able to talk to people in a way in which Hillary Clinton can't."

Obama attracted much attention this week with her powerful speech in New Hampshire, where she made a passionate endorsement for Clinton while denouncing Republican presidential candidate Donald Trump's "shocking and demeaning" comments about women.

Michelle Obama attacks Trump's comments about women 1:36

Clinton, at least according to her Twitter account, couldn't have been more pleased, tweeting out excerpts of her speech, saying that she was "in awe" and thanking Obama for "putting into words what's in so many of our hearts."

.<a href="https://twitter.com/FLOTUS">@FLOTUS</a>, I'm in awe. Thanks for putting into words what's in so many of our hearts. -H —@HillaryClinton

It's the second time a Michelle Obama speech for Clinton has attracted widespread attention. At the Democratic convention in July, she heaped praise on Clinton in a well-received and emotional address.

President's wife gives an impassioned speech to support Hillary Clinton 15:52

But Kate Andersen Brower, author of First Women: The Grace and Power of America's Modern First Ladies, said that just because Obama is stumping for Clinton, it doesn't mean they have a warm relationship. In fact, as Brower wrote in her book, the scars from the 2008 primaries remain "shockingly fresh."

According to a former Obama aide, when Michelle Obama views the Clintons, "she kind of looks down her nose at them," said Brower. She feels that the Clintons could have accomplished a lot more, if not for Bill Clinton's indiscretions, and that they're both overly ambitious, Brower added.

'She hates Donald Trump'

Yet despite those tensions, Brower said, Obama still wants Clinton to be president.

"I think this election is so charged, I don't think it's disingenuous when you see Michelle Obama out there passionately advocating [for Clinton], because she hates Donald Trump," Brower said.

Democratic presidential candidate Hillary Clinton and Michelle Obama have reportedly had a tense relationship since the 2008 primaries. (Andrew Harnik/Associated Press)

Obama is also someone who doesn't enjoy campaigning, even for her husband, which lends her a political air of authenticity when she heads out on the trail, Brower said.

"We pay attention when she does. So you don't see her all the time, but when you see her she usually has something to say, and I think people like that."

"She is someone who people of many different ages and various backgrounds connect with, which in a way isn't so much the case with Hillary Clinton," said Katherine Sibley, director of the American Studies Program at Saint Joseph's University and editor of A Companion to First Ladies.

People see her on television, in popular culture, including her popular appearances on Ellen or her carpool karaoke on The Late Late Show with James Corden.

"I think there is this much broader appeal that she has," said Sibley. "She can help by being someone who is reaching out to this broader audience, the youthful audience especially."

The Clinton team is certainly aware of Obama's effectiveness and ability to draw people into the campaign, especially young voters. At a recent rally in Pittsburgh where Obama was the featured speaker, many Clinton campaign volunteers armed with clip boards waded through the crowd gathering information.

A Clinton campaign volunteer works in the crowd at a rally in Pittsburgh. (Meagan Fitzpatrick/CBC)

"I thought it was very powerful," said Mahlon Chase, 18, referring to Obama's speech at the rally. "I'm not that into politics but at the same time … I'm definitely inspired to vote."

"She told me about all the accomplishments of Hillary Clinton and what she's done for this country already. So I feel like she's qualified as a candidate."

'Michelle is just so influential'

"Michelle is just so influential, especially with the young community," said Marsaude Edwards, 18. "I think this is very important, her coming out and representing, and sharing her opinion and influencing everyone else."

Obama is a popular figure who enjoys high approval ratings and has won praise for her series of apolitical policy initiatives. They include her Let's Move campaign to tackle childhood obesity, Let Girls Learn, an initiative to educate adolescent girls around the world, and Joining Forces, with Jill Biden, to support veterans and their families.

Yet she got off to a rocky start with the public, even before entering the White House. In 2008, she sparked condemnation, particularly from the political right, when she said during the primaries that for "the first time in my adult life, I am really proud of my country, because it feels like hope is finally making a comeback."

"In 2008, she was really polarizing," said Brower. "She's really turned that around in an incredible way."

"I think she just got used to having her words dissected and yet … she can maintain this level of reality when she speaks, even though she knows everything is going to be dissected."

Michelle Obama says she has no plans to make her own run for the presidency. (Mel Evans/Associated Press)

As for her political future, many Democrats would love to see Obama take her own run for the White House. But it's an idea she has nixed..

"Many first ladies have described the White House as a prison, and she is one of them," said Brower. "I don't think she's particularly happy there."

Her husband may have revealed the best clue about his wife's future political ambitions when he said: "There are three things that are certain in life: death, taxes, and Michelle is not running for president."