Michelle Obama: Her four-year evolution

Michelle Obama at a walk-through of her 2012 convention speech

In 2008, some controversial remarks threatened to derail her husband's election campaign. Yet four years later, Michelle Obama is widely regarded as a political star.

When Michelle Obama takes the stage on Tuesday night at the Democratic National Convention, she does so as a popular public figure with bipartisan appeal.

But four years ago, on the campaign trail in 2008, Mrs Obama was labelled by her critics as angry, bitter and militant.

Statements from her thesis at Princeton, in which she examined the effect of being black at a predominantly white college, saw her portrayed as obsessed with race.

Campaigning for her husband in Wisconsin, she told supporters that she was proud of America "for the first time in my adult life". It was a comment that dogged her throughout the summer.

"She was depicted as being unpatriotic and un-American," says Mia Moody-Ramirez, a professor of journalism, PR and new media at Baylor University in Waco, Texas.

But then, at the Democratic convention in Denver, she gave a speech introducing a new version of Michelle Obama - one that would help portray her as a loving wife, mother and daughter, and would help reshape her image into the one that is much-loved today.

Michelle Obama's prominent - and some say distracting - role on the campaign trail in 2008 came despite her initial displeasure at her husband's political ambition.

"There is a pattern - at different points in their lives, she is very sceptical and hesitant about entering into political races, and once she does [back her husband], she's his strongest advocate," say Jodi Kantor, a reporter for the New York Times and author of The Obamas.

And in the campaign of 2008, Michelle Obama was bringing the full force of her personality and her training to bear.

"People who have known her for a long time say she is a naturally frank, forceful person. She's a Harvard-trained lawyer. Her tendency was to go in and argue her husband's case," says Kantor.

But the effect on the public was less than positive, in part because political wives normally take on a less prominent role, and in part because Michelle Obama was forging new ground.

"There was this perception that she was a problem for the campaign," says Bonnie Dow, an associate professor of communication at Vanderbilt University.

"The outlines of that problem are so easily tied to negative stereotypes about African-American women - that they are matriarchal, pushy loud."

When it became clear that Mrs Obama's campaign style was a detriment to her husband, she and the campaign team sought to modify her image, from dogged lawyer to loving mother and wife.

"Obama advisers put a lot of effort into tailoring her image. They brought through maternal warmth and left behind her bluntness," says Kantor.

"I don't think the Michelle Obama we see on the campaign trail is fake, she's a highly edited version."

That tailoring began over the summer of 2008, and in some ways made its public debut at the convention in Denver. Introduced by her brother, Craig Robinson, Michelle Obama gave a speech about her husband that her supporters thought was warm, funny and loving .

"When we go into the DNC... and she gives this speech, introducing the president by saying 'I stand here as a wife, mother, daughter, sister', you see the beginnings of a very purposeful rehabilitation of this image," says Dow.

The convention speech helped accelerate Mrs Obama's popularity, and by the time of her husband's inauguration in January of 2009 she was on her way to becoming a media darling.

"On inauguration day she stepped out in eye-catching dresses, and her children looked so adorable in their little winter coats and tights," says Katherine Jellison, who has studied first ladies as a professor of history at Ohio University.

"She came across on that occasion as the kind of first lady that I think America is largely most comfortable with - the devoted wife and mother."

While Barack Obama's approval ratings began dropping within his first months in office, Michelle Obama's have continued to rise. When Gallup last measured the two Obamas head-to-head, in May 2012, Mrs Obama had a 66% approval rating, while her husband had 52% approval.

Once she became the first lady, she adopted projects with a maternal focus, a role embraced by the women who came before her.

"Every issue that she takes up is linked to children and to family - she doesn't even do that many issues that are specifically linked to women who aren't mothering," says Dow.

While former librarian Laura Bush promoted literacy, Mrs Obama worked on projects for military families and Let's Move, an initiative to fight childhood obesity. In that role, she has planted a vegetable garden in the White House, appeared on reality TV to promote weight loss and written a book about her gardening success.

"Once she got into office, she realised that this was about [Barack Obama] and it had to just be about him, or there was going to be a problem. She focused on issues that did not put her in the forefront and did not overshadow him," says Allison Samuels, a senior writer at Newsweek and author of What Would Michelle Obama Do?

Image caption The Obamas have reprised their 2008 campaign double act this time around

This election cycle, Democrats hope Michelle Obama can be a powerful weapon in the re-election arsenal, not a liability to be managed.

Some Republicans number among her fans but over the years her detractors have found fault with her clothes, her projects and her views.

Her speech on Tuesday in 2012 will seek to convince the few and crucial undecided voters that her husband's plan is the best for the country. And in an era when many families are worried about paying the bills, Samuels says Michelle Obama's message may resonate more than her Republican counterpart, Ann Romney.

"Ann Romney was talking in theory, whereas Michelle had to go back to work quickly after every child because they didn't have the money," she says. "When she talks about the issues of women and poor women, it's more authentic."

To counter that argument, her critics would point to reports that she spent $6,800 on a jacket when Americans are finding it hard to make ends meet. And Ann Romney, who has fought breast cancer and multiple sclerosis, appealed directly to women when she gave a speech that ignited the Republican party's gala last week.

Just because Michelle Obama is no longer a political lightning rod, it doesn't mean she has lost her political pull.

"Even though she is able to look like she is above the fray, of course she is political. She's out there campaigning, she's out there fundraising, she's one of the most popular political figures in the country," says Kantor.

This week, she will try to use the power of her popularity to bolster her husband's hopes of winning a second term in office.

"Remember the contradiction of first lady-hood is that the less overly political a first lady seems, the more politically effective she is," says Kantor.