Bill Clinton, the last two-term Democratic president, will urge US voters Wednesday to let Barack Obama lead America back to prosperity, in a highly-anticipated convention speech.

The hugely popular figure who presided over a period of strong growth, low unemployment and budget surpluses will make the economic case for re-electing Obama and spurning his Republican opponent on November 6, Mitt Romney.

In a "Clear Choice" Obama campaign advert a few weeks ago, Clinton previewed the message we can expect on Wednesday night when the silky political veteran retakes the spotlight in a packed convention hall in Charlotte, North Carolina.

"This election to me is about which candidate is more likely to return us to full employment. This is a clear choice," he says in the ad.

"The Republican plan is cut more taxes on upper income people and go back to deregulation. That's what got us into trouble in the first place.

"President Obama has a plan to rebuild America from the bottom up, investing in innovation, education and job training. It only works if there is a strong middle class. That's what happened when I was president. We need to keep going with his plan."

Clinton's economic pitch follows an impassioned plea by First Lady Michelle Obama on Tuesday's opening night of the Democratic convention in which she recalled with strong emotion the hardships of her husband's amazing journey.

"Barack was raised by a single mother who struggled to pay the bills, and by grandparents who stepped in when she needed help," she said. "Barack knows what it means when a family struggles. So in the end, for Barack, these issues aren't political -- they're personal."

Throughout the opening night, speaker after speaker portrayed Romney, who was born into privilege and amassed a $250 million fortune running private equity firm Bain Capital, as a ruthless corporate raider who just didn't get it.

"Barack knows the American Dream because he's lived it -- and he wants everyone in this country to have that same opportunity, no matter who we are, or where we're from, or what we look like, or who we love," the First Lady said.

The arena in Charlotte, packed with thousands of Democratic delegates and Obama supporters, erupted whenever Romney came under attack, with chants of "Four more years! "Four more years!" echoing across the auditorium.

Democrats have made hay out of the wealth issue throughout the presidential campaign, attacking Romney for keeping much of his estimated $250 million fortune in offshore havens and asking why he will not release more tax returns.

The Republicans have in turn accused Obama of disparaging success and waging "class warfare," while failing to spur a more robust economic recovery or bring the unemployment rate down from a stubborn 8.3 percent.

With less than nine weeks until election day, the race between Obama and the former Massachusetts governor remains too close to call and will hinge on the results in a handful of swing states like Virginia, Ohio and Florida.

Clinton's economic pitch on Wednesday night could be a hard sell to many Americans still feeling the affects of a "Great Recession" that continues to stall growth and sour the investment mood.

But the former president, who often infuriated Americans but has retained their affection, is masterful at courting critical audiences that Obama struggles to reach, including white, working class men in swing states.

Romney's campaign has leveraged Clinton's popularity by contrasting the prosperity of the 1990s with Obama's America, and accuses the president of rolling back his predecessor's welfare reform.

A sign perhaps that Bill, aged 66 and 12 years out of office, isn't the star draw he once was, NBC has decided to show the opening game of the NFL American Football season, the Dallas Cowboys versus the New York Giants, instead.

Obama flies to Charlotte on Wednesday on the eve of a nomination acceptance speech in which he will try to persuade the American people to give him a second term despite the grim economic backdrop.

Plans for the president to address a massive 70,000-plus-seater outdoor football stadium as he seeks to rekindle his 2008 convention magic were scrapped due to the threat of severe weather.

His speech to formally accept the Democratic presidential nomination was moved instead to the 20,000-seater Time Warner Cable Arena, the same stage used by the other speakers throughout the three-night convention.