To deafening chants of "four more years" from delegates at the Democratic Party convention, United States first lady Michelle Obama delivered an impassioned appeal for her husband's re-election, saying he has "so much more to do" as President.

"We can surely give everyone a chance at that great American dream," she told the ecstatic delegates in the address in which she admitted that President Barack Obama was aware that "we are playing a long game here, and that change is hard".

She did not give a single mention to Obama's Republican challenger, Mitt Romney, who has derided the President's failure to produce the "hope and change" he promised in 2008.

Yet much of her speech implicitly rebutted criticisms from Romney, the multimillionaire former Massachusetts Governor portrayed as an elitist by earlier speakers on the convention's first day in Charlotte, North Carolina.

"Success isn't about how much money you make, it's about the difference you make in people's lives," she said in a swipe against Romney. Obama "knows the American dream because he's lived it".

Michelle Obama, returning to personal stories she first recounted at the Democratic convention four years ago, described her husband as a man raised in hardship with the courage to stand up for the values inherited from his family.

In front of an audience including many African Americans - in contrast to last week's almost exclusively white Republican convention in Florida - she defended his controversial healthcare reform as "the right thing to do".

"When it comes to the health of our families, Barack refused to listen to all those folks who told him to leave health reform for another day, another President. No one should ever go broke because of an accident or an illness."

Michelle Obama's warm endorsement of her husband as a man who understands the problems of average Americans followed an evening of harsh words for Romney and his running mate, Congressman Paul Ryan.

The pair were accused by the Latino Mayor of San Antonio, Julian Castro, of favouring policies that would not just pummel but "dismantle the middle class". Castro energised the crowd by telling delegates that "Mitt Romney quite simply doesn't get it".

"A few months ago he visited a university in Ohio and gave the students there a little entrepreneurial advice. 'Start a business,' he said. But how? 'Borrow money if you have to from your parents.'

"Gee," Castro added sarcastically. "Why didn't I think of that?"

Delegates cheered as Castro described as an "extreme makeover, and it ain't pretty" Romney's shift to the right to attract the social conservative wing of the Republican Party.

"To Romney we're going to say no," he proclaimed.

Castro and others also countered the central argument of the Romney campaign, that Americans are worse off now than four years ago and that Obama should be voted out of office in November to allow former CEO Romney to bring about economic recovery.

Deval Patrick, the black Governor of Massachusetts, demolished his predecessor's record, saying that he had cut education "deeper than anywhere else in America".

Romney "talks about all the things he's fixed. I can tell you Massachusetts was not one of them", said Patrick.

He praised the President for adopting the health reforms, assassinating Osama bin Laden, saving the car industry from extinction, creating jobs and repealing the ban on gays serving in the military.

"The list of accomplishments is long, impressive, and barely told."

Pointing out that Obama had also had to contend with a bitterly divided Congress which had deliberately obstructed the Democratic President's initiatives, Patrick gave delegates a rallying cry, saying: "I for one won't stand by and let him be bullied out of office, and neither should you."

But one of the longest ovations was reserved for Lilly Ledbetter, an Alabama woman whose suit for fair pay became the basis of the first law signed by Obama.

After pointing out that women still earn only 77c for every dollar earned by a man, she added to rapturous applause: "Maybe 23c doesn't sound like a lot to someone with a Swiss bank account and a Cayman Island investment." It was a reference to Romney's offshore bank accounts, and it was devastating.