Democrats kick off their 2012 convention on Tuesday, with First Lady Michelle Obama firing the first shots of a three-day salvo aimed at breaking the electoral deadlock with Republicans.

With just 63 days until Americans decide who will occupy the Oval Office for the next four years, the First Lady will make the case it should be her husband, the country's first black president.

Michelle, who is more popular than the president, is expected to woo women voters, an effort to eke out any advantage in a race marked by too-close-to-call polls and a dearth of undecided voters.

Her speech comes four years after she vowed - before a stadium full of delegates in Denver, Colorado - that Barack Obama, despite his "funny name," would make an "extraordinary president".

With economic malaise casting serious doubt on that claim, Democrats gather in Charlotte in the battleground state of North Carolina for an event that provides a vital nationwide platform seen just once every four years.

Their first task will be to counter the cutting Republican charge that, though his election was historic and rightly celebrated, Obama's presidency has been a bust.

After last week's Republican convention, some Democrats spot a fresh opening to win that argument.

Hopes in the Romney camp that their Tampa-based convention would help him nose ahead in the White House race were dimmed by new polling data by Gallup, showing little change.

Forty percent of adults asked over the last three days said the convention had made them more likely to vote for Romney in November's election, but a similar 38 percent said events in Florida made them less likely to back him.

Obama led the man trying to deny him a second term by 47 percent to 46 percent in Gallup's latest daily tracking poll, which has rarely strayed beyond the margin of error since Romney became the presumptive nominee in April.

Though cheered by polling data, Democrats were also trying to put out the latest campaign brush fire, after several top party figures struggled to answer on Sunday whether Americans were better off after four years of Obama.

"America is better off today than they (Republicans) left us," said Vice-President Joe Biden, campaigning in Michigan.

"Let me just sum it up this way ... Osama Bin Laden is dead and General Motors is alive."

But Republican vice-presidential hopeful Paul Ryan, in a raid into North Carolina ahead of the Democratic convention, drew comparisons between Obama and one-term Democratic president Jimmy Carter, who was felled by a poor economy.

"The president can say a lot of things, and he will, but he can't tell you that you're better off. Simply put, the Jimmy Carter years look like the good old days compared to where we are right now."

In a sign of how close the Obama campaign believes the race remains, the president continued a four-day "Road to Charlotte" tour taking in territory that will decide November's election.

Agence France-Presse