President Barack Obama speaks during a campaign event at Norfolk State University in Norfolk, Virginia.

Charlotte, North Carolina - Democrats launch their case for Barack Obama's re-election at their party convention on Tuesday, looking to draw a sharp contrast with Republican Mitt Romney and convince voters that the US president deserves four more years to fix the economy.

A speech by First Lady Michelle Obama caps the opening night of the three-day gathering in Charlotte, North Carolina, which concludes with Obama's acceptance of the nomination in an address on Thursday in a 74 000-seat downtown football stadium.

The convention gives Obama a chance to recapture the political spotlight from Romney and Republicans, who used their gathering last week to repeatedly attack Obama's economic leadership.

Republicans stayed on the offensive ahead on Tuesday, criticising Obama for telling a Colorado television reporter that he would give himself a grade of “incomplete” for his first term.

“Four years into a presidency and it's incomplete? The president is asking people just to be patient with him?” Republican vice-presidential nominee Paul Ryan said on CBS's This Morning.

“The kind of recession we had, we should be bouncing out of it,” Ryan said. “We're not creating jobs at near the pace we could. That's why we're offering big solutions for the big problems we have today.”

Obama campaign spokesperson Jen Psaki said the president was simply saying more work was needed to turn around a stumbling economy with a persistently high 8.3 percent unemployment rate.

“I think an incomplete and a desire to do more is far better than a failing grade,” she told reporters on Air Force One as Obama headed to a campaign event in Norfolk, Virginia.

Obama's economic argument got a little tougher on Tuesday. New surveys showed US manufacturing shrank at its sharpest clip in more than three years last month, while exports and hiring in the sector also slumped.

As the convention opens, the task for Obama and his allies will be to persuade voters disappointed by his first White House term that things will be better the second time around, while portraying the budget-slashing economic remedies offered by Romney and Ryan as unacceptable alternatives.

While Republicans focused on attacking Obama and helping voters get to know Romney during their convention, the Democrats' goal will be to keep up voter enthusiasm for an incumbent in tough economic times.

They said they will highlight Obama's successes during his first term - from ordering the mission that killed al-Qaeda leader Osama bin Laden to the bailout of the auto industry - while reminding voters of the difficulties Obama faced when he took office.

Organisers were nervously watching the weather. Scattered thunderstorms were predicted for Thursday night when Obama is scheduled to give his speech in an open-air stadium. If necessary, the speech could be moved back to the much smaller basketball arena that hosts the first two sessions.

Romney and Obama are running close in opinion polls ahead of the November 6 election, but Obama hopes to get more of a convention “bounce” in polls than Romney, who gained a few percentage points at most from the Tampa, Florida, event.

A Reuters/Ipsos online poll on Tuesday gave Romney a one-point edge on Obama, 46 percent to 45 percent, a slight improvement from Obama's 4-point lead before the Republican convention began last week.

But a Gallup poll on Monday showed Romney's speech last week got the worst scores of any convention acceptance address going back to 1996, when it began measuring them. Thirty-eight percent rated the speech as excellent or good; the previous worst had been Republican John McCain's in 2008, at 47 percent.

Democrats plan to use their convention to highlight the party's diversity, featuring a lineup of black, Hispanic and young speakers to appeal to the voting blocs that helped propel Obama to a comfortable victory in 2008.

The keynote speaker on Tuesday will be San Antonio Mayor Julian Castro, a Hispanic rising star in the party.

Michelle Obama's speech will counter a successful Republican convention appearance last week by Romney's wife, Ann, who helped present a softer and more personal side of Romney to voters, who polls show have had a hard time warming up to the sometimes stiff former Massachusetts governor.

“I think the first lady plays a special role because she will have personal perspective on the president's leadership - his grit and determination during a challenging time for our nation,” Obama campaign spokesperson Ben LaBolt said.

“She is a character witness for the president and someone who can address how he has made decisions as the nation has confronted these challenges,” he said.

Obama planned to watch his wife's speech from the White House with his daughters. “I'm going to try not to let them see daddy cry,” he told supporters at the Norfolk State University rally. “Because when Michelle starts talking, I start getting all misty.”

Romney will give Obama the political spotlight and stay off the campaign trail for most of this week. He spent Tuesday in Woodstock, Vermont, preparing for the debates that begin on October 3.

The opening session will convene at 5pm EDT (21h00 GMT). Democrats will approve their non-binding party platform, which includes calls for higher taxes on the wealthiest Americans and support for same-sex marriage and a woman's right to abortion.

Former president Jimmy Carter will be featured in a video on Tuesday. Former president Bill Clinton will highlight Wednesday's slate of speakers in an address that could remind voters of his Democratic-led economic growth in the 1990s while appealing to the white working-class Democrats that Obama has had difficulty winning over.

The Obama campaign also plans to use the convention and Obama's speech on Thursday as an organising tool to help them in North Carolina, a battleground state that Obama won in 2008 but polls show is too close to call this time around. - Reuters