CHARLOTTE, North Carolina (Reuters) - Democrats launch their case for U.S. President Barack Obama's re-election at his nominating convention on Tuesday, looking to draw a sharp contrast with Republican rival Mitt Romney and convince voters that Obama has the more sensible plan for economic recovery.

First lady Michelle Obama's appearance highlights the opening night of the three-day Democratic gathering in Charlotte, North Carolina, which concludes with Obama's acceptance speech on Thursday to more than 65,000 supporters in a downtown football stadium.

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

The task for Obama and his allies will be to convince 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 his running mate, Congressman Paul Ryan, as unacceptable alternatives.

"There are two paths ahead and a crystal clear choice before us," said Democratic National Convention Chairman Antonio Villaraigosa, the mayor of Los Angeles.

Romney and Obama have been running fairly even 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 event.

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 black, young and Hispanic speakers to appeal to the voting blocs that helped 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 Tuesday 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," said Obama campaign spokesman Ben LaBolt.

'A CHARACTER WITNESS'

"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.

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 organizing 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.

Democrats will use the convention as a backdrop to spell out Obama's successes during his first term, from ordering of the mission that killed al Qaeda leader Osama bin Laden to the bailout of the auto industry and the healthcare overhaul.

But the two camps squabbled again on Monday over the question of whether voters were better off now than when Obama took office nearly four years ago.

Republicans said the slow economic recovery and 8.3 percent unemployment showed Obama had flunked that basic political test. But Democrats were adamant that there had been improvement under Obama.

"Absolutely," Stephanie Cutter, Obama's deputy campaign manager, said on NBC's "Today" show.

"By any measure the country has moved forward over the last four years. It might not be as fast as some people would've hoped. The president agrees with that," she said.