Michelle Obama, the Harvard-educated African-American wife of the Democratic presidential hopeful Barack Obama, announced yesterday that she was quitting her job to join her husband on the campaign trail.

Mrs Obama’s entrance on to the national stage is a careful ploy to cut into Hillary Clinton’s overwhelming strength among women voters – the dominant constituency in the Democratic primaries. It also served to accustom America to the idea that an Obama victory in 2008 would give the country its first black First Lady.

Mrs Obama, 43, a glamorous, 5ft 11in Chicago lawyer, appeared on the front pages of bothUSA TodayandThe Washington Post clearly intent on preempting her husband’s inevitable fall to earth after months of adulation from adoring crowds. His record-breaking primary fundraising effort, which has so far eclipsed Mrs Clinton’s, has helped to give him the image of a super-star who can work political miracles. “He’s not the next messiah who’s going to fix it all,” Mrs Obama said. “He is going to stumble. He is going to make mistakes.”

In recent weeks, Mrs Obama has begun making campaign appearances in small “town hall” settings in some key primary states, including a visit to New Hampshire this week. With the campaign clearly anxious to manage expectations after her husband’s meteoric rise and charismatic image – two years ago he was an unknown Illinois state senator – Mrs Obama has been trying to persuade people that he also has domestic feet of clay, telling anecdotes about his failure to put the butter away, an inability to make beds – and even stories about his dirty socks.

One critic, the poison-penned New York Times columnist Maureen Dowd, called such talk “emasculating”, and in recent days Mrs Obama has dropped references to butter, beds and dirty laundry.

The Obama campaign has introduced his family – the couple have two daughters, Malia, 8, and Sasha, 5 – only sparingly. But the new push by Mrs Obama, which comes with her first fundraising letter, reflects what an important role political spouses can play in a campaign, and how several have already made a big splash.

John Edwards, a rival for the Democratic nomination, had a week of unrivalled and uncritical coverage after his wife Elizabeth, also a lawyer, announced a return of breast cancer. Ann Romney, the wife of the Republican candidate Mitt Romney, has spoken movingly about her multiple sclerosis.

And, of course, there is another spouse named Bill Clinton, who has been shaking the fundraising trees to great effect and playing the humble First Husband.

Mrs Obama, an exercise addict, admits that her decision to quit her $275,000-a-year job as a vice-president at the University of Chicago Hospitals felt “very odd” and “disconcerting” after a life striving to climb the professional ladder.

But it will sit well with many voters, especially women who find professional mothers off-putting. National polls show that Americans also favour First Ladies in the Laura Bush mould rather than hyper-ambitious career women.

Mrs Obama grew up in a tiny working-class home on the south side of Chicago, its doors and windows fortified with iron bars. Disciplined and brilliant, she followed her brother to Princeton, before attending Harvard Law School. For the next three years, she worked as a lawyer in the Chicago firm Sidley Austin, where she was assigned to mentor a summer intern named Barack Obama. They married in 1992.

She played a crucial role in her husband’s political rise in Chicago, one of the strongest and most influential black constituencies in America. His father was Kenyan, mother a white Kansan, and many blacks refused to accept him as a fellow African American. But in his 2004 Democrat Senate primary Mr Obama received the support of the city’s black business leaders, many of whom had close ties to his wife.

Mrs Obama insists she is unscripted and independent from her husband, which carries risks. “Do you expect me to hold my tongue?” she says when asked what happens when her husband wants input on policy issues. Like John Kerry before him, whose wife damaged his campaign with her loose tongue, Mr Obama must hope that at key moments his wife really does hold hers.

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