She implies that Romney, who had a privileged upbringing, can’t relate when she tells middle-class voters that President Barack Obama understands their economic struggles because he has struggled too. And she suggests Romney would have other priorities when she says her husband’s empathy will result in a second-term agenda focused squarely on middle-class economic security.

The first lady will make her case to millions of Americans on Tuesday when she headlines the first night of the Democratic Party’s national convention, where two days later her husband will accept the party’s presidential nomination for a second time. Her high-profile appearance underscores her key role in his re-election bid: chief defender of his character and leader in efforts to validate the direction he is taking the country.

Once the reluctant political spouse, she has embraced that mission to sell her husband anew throughout the summer while raising money for the campaign and speaking at rallies in battleground states.

These days, Mrs. Obama’s speeches are peppered with references to the president’s upbringing in Hawaii, where he was raised by a single mother and his grandparents. She talks about the student loans he took out to pay for college and the years it took to pay them back.

When Romney accused Obama of running a “campaign of hate,” the first lady delivered Obama’s strongest counterpoint — without mentioning the Republican candidate.

“We all know who my husband is, don’t we? And we all know what he stands for,” she said, standing alongside the president at a campaign rally in Iowa.

Key to Mrs. Obama’s campaign strategy is maintaining her own personal appeal.

Anita McBride, who served as first lady Laura Bush’s chief of staff, said that means staying away from the vitriol that has permeated the White House campaign.

“There are plenty of attack dogs in this campaign,” McBride said. “She doesn’t need to be one of them.”

In many ways, the first lady’s challenge Tuesday night will be more difficult than it was when she spoke at the 2008 Democratic convention. Back then, her mission was to vouch for her husband’s personal qualities. This time around, she also has to persuade voters to stick with him amid high unemployment and sluggish economic growth.

Many Americans didn’t know Mrs. Obama and some viewed her suspiciously ahead of the 2008 convention. Republicans had questioned her patriotism throughout the campaign because she told voters during the primary that “for the first time in my adult lifetime, I’m really proud of my country.”

Her convention speech sought to put those issues to rest. She declared “I love this country” and used personal stories about her marriage to assure voters they had nothing to fear about her and her husband’s values.

Since moving into the White House, Mrs. Obama has focused on tackling childhood obesity and assisting military families. She’s largely steered clear of her husband’s political battles, at least in public.

But behind the scenes, she’s a sounding board for her husband on pressing policy matters. She also has increasingly promoted his health care overhaul after it was upheld by the Supreme Court.

Aides say she will sprinkle her remarks Tuesday with a defense of the president’s policies, including the health care law and the Lilly Ledbetter Fair Pay Act, which was the first legislation Obama signed into law. The act makes it easier for women to sue for equal pay if they earn less than their male counterparts. Obama has made the law a key part of his election year appeal to women, who could give him an edge over Romney in a tight race.

The first lady arrived in Charlotte on Monday and informally rehearsed at the Time Warner Cable Arena. She also taped interviews for entertainment programs that will air before her speech.

Mrs. Obama is staying in Charlotte during the three-day convention and will focus on shoring up support for her husband among key constituencies. She plans to speak to the party’s African-American, Hispanic and women’s caucuses and address a gay and lesbian luncheon. Along with the vice president’s wife, Jill Biden, the first lady will also participate in an armed services event Thursday and put together care packages for U.S. troops serving overseas.

Mrs. Obama will join a crowd of up to 74,000 people at an outdoor football stadium in Charlotte on Thursday night when the president formally accepts the Democratic nomination. The first lady is not expected to have a speaking role that night, but she, and possibly her young daughters, will join the president on stage, leaving voters with fresh images of the photogenic family.

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