PHILIP DELVES BROUGHTON: Does Michelle Obama deserve to be America's First Lady?

But she left much to be done in what has become the central obstacle to Barack Obama's winning the presidency: reassuring the country that she and her family understand ordinary Americans.

The first task for Mrs Obama was to shed a reputation for acerbity and bitterness which has clung to her since the early months of the year.

Michelle Obama, wife of Democratic presidential candidate Barack Obama

She has yet to live down her remark that her husband's campaign marked the first time in her adult life when she was 'proud of my country'.

Critics have labelled her yet another haughty liberal, ungrateful for her opportunities and dismissive of anyone who disagrees with her.

If she could only be proud of America when her husband was vying for the Presidency, did she really deserve to be First Lady?

Hence the theme of her speech: A profound love of family and country. She spoke of growing up poor in Chicago and watching her father, who spent 30 years working for the city's water treatment plant, stoically coping with MS.

She said she shared an understanding of the American dream with 'people who work the day shift, kiss their kids goodnight, and head out for the night shift'. She was tearful as she spoke of how she and her husband were driven by 'a simple belief that the world as it is just won't do'.

Her message of triumph against the odds and her smooth delivery was worthy of the Obamas' close friend and supporter Oprah Winfrey.

Michelle with daughters Sasha, 7, and Malia, 10 at the Democratic Convention

But what was left out was just as telling: Any mention by name of the universities she attended, Princeton and Harvard, and her highly paid career as a corporate lawyer.

The last thing the Obama campaign wants is for the couple to come across as 'Clinton Lite', another over-educated pair of liberal lawyers planning to run the White House as a team. Last night's speech was part of a broader strategy to counter the perception that Obama is an elitist.

During the primary race against Hillary Clinton, Obama struggled to win over blue-collar and middleclass white voters. His Kenyan father, his upbringing by a single mother in Hawaii and his Ivy-League education seemed odd to voters who prefer a more conventional story for their politicians. John McCain's war hero-turned-politician was instantly recognisable.

Michelle Obama's speech, which rooted her in the black working-class of Chicago, was intended to anchor her husband in a tale of hard work, honesty and upward mobility through education.

Chat show host Oprah Winfrey is a supporter of Barack Obama

When Barack appeared on a large video screen at the end of the speech, he was sitting with a white family in that whitest of places, Kansas City, Missouri.

He has given up the stadium speeches which marked his primary campaign. He tells voters how he and his wife struggled to pay off their student loans and fretted about childcare.

It is an attempt to make a distinction between the self-made Obamas and Republican candidate John McCain, and his wife who inherited a large fortune.

Last week's poll by U.S.A Today newspaper found 52 per cent of voters believed the candidates' spouses were important in deciding who to vote for. In a tight contest the performance of Michelle Obama and Cindy McCain will be critical.

For a candidate who remains as mysterious as Obama, Michelle's role will be vital. Hence the effort which has gone into softening her image  -  appearing on magazine covers, and discussing everything from shopping for toilet paper to raising her daughters Malia, ten, and Sasha, seven.

It is a maddening fact for Democrats that true American aristocrat President Bush managed to appear down-to-earth in his two election campaigns.

Yet the Obamas, a black couple who did genuinely work their way up the system, are struggling to rid themselves of the elitist tag.

Michelle did some sterling work at the convention, but the Obamas' challenge remains a huge one.