FIRST Lady Michelle Obama acknowledged that the change her husband Barack Obama championed in his White House campaign four years ago has proven difficult but urged voters to give him four more years to fix the US economy.

"We are playing a long game here, and that change is hard, and change is slow, and it never happens all at once," she told the Democratic National Convention in Charlotte, North Carolina. "But eventually we get there. We always do."

The popular first lady was the highest-profile advocate for her husband in the first of three days of speeches that will conclude with Mr Obama's address tonight to accept the Democratic nomination to face Mitt Romney on November 6.

In a race that is too close to call nine weeks before Americans vote, Mr Obama is vulnerable to the challenge from Republican nominee Mr Romney due to a sluggish economy and 8.3% unemployment.

The President is trying to use his convention to recapture the magic that carried him to victory in 2008 but admitted he would give himself a grade of "incomplete" for his first term.

There were what appeared to be subtle digs from Mrs Obama at her husband's Republican rival.

"For Barack, success isn't about how much money you make, it's about the difference you make in people's lives," she said, perhaps a reference to multimillionaire Mr Romney.

A host of speakers at the gathering in Charlotte attacked Mr Romney for his business record, refusal to release more tax returns and for spearheading a Republican "war on women".

The Democrats even choreographed a swipe at the former Massachusetts governor from beyond the grave, by playing a video of late Senator Ted Kennedy getting the better of Mr Romney during a debate in the 1994 election campaign for Mr Kennedy's Senate seat.

The Democrats highlighted Mr Obama's successes during his first term – from ordering the mission that killed al Qaeda leader Osama bin Laden to the bailout of the car industry – while reminding voters of the difficulties he faced when he took office.

Mrs Obama urged party activists to rally around the president.

She said: "We must work like never before, and we must once again come together and stand together for the man we can trust to keep moving this great country forward, my husband, our president, Barack Obama."