(Reuters) - Democrats defended President Barack Obama's handling of the struggling U.S. economy on Tuesday and urged voters to give him four more years as they opened their national convention with sharp criticism of Republican Mitt Romney.
A speech by first lady Michelle Obama is the main event at the start of the three-day gathering in Charlotte, 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 seize the political spotlight from Romney and Republicans, who used their gathering last week to repeatedly attack Obama's economic leadership.
In a fiery speech, Newark Mayor Cory Booker fought back against Republican complaints about Obama's plans to raise taxes on the richest Americans.
"Being asked to pay your fair share isn't class warfare. It's patriotism," said Booker, a rising star in the party.
Obama enters the convention vulnerable over his handling of the economy, which is struggling under the weight of an 8.3 percent jobless rate.
Democratic speakers argued Obama has done well in bringing the country back from a possible depression when he took over in January 2009 and deserves another chance.
"Four years ago, America stood on the brink of a depression," Julian Castro, mayor of the Texas city of San Antonio, will say, according to excerpts of his speech due to be delivered later in the evening.
"Despite incredible odds and united Republican opposition, our president took action. And now we've seen 4.5 million new jobs. He knows better than anyone that there's more hard work to do. But we're making progress," he will say in the keynote speech.
Republicans stayed on the offensive, criticizing 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's economic argument got a little tougher on Tuesday. New surveys showed U.S. manufacturing shrank at its sharpest clip in more than three years last month, while exports and hiring in the sector also slumped.
MICHELLE OBAMA SPEECH
Democrats plan to use their convention to highlight the party's diversity. The opening night featured a lineup of black, women, Hispanic and young speakers to appeal to the voting blocs that helped propel Obama to a comfortable victory in 2008.
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.
"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," said Obama campaign spokesman Ben LaBolt.
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."
The Democrats approved their non-binding party platform, which included calls for higher taxes on the wealthiest Americans and support for same-sex marriage and a woman's right to abortion.
Former President Bill Clinton, who presided over economic boom times in his 1990s White House years, is the main Wednesday speaker. A campaign TV ad featuring Clinton talking up Obama's leadership has been playing on Charlotte stations.
The Democrats 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.
Organizers 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. But Obama campaign officials did not want to do that unless absolutely necessary.
Romney and Obama are running about even in opinion polls before 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 1-point edge on Obama, 46 percent to 45 percent, a slight improvement from Obama's 4-point lead before the Republican National Convention began last week.
Romney will give Obama the political spotlight and stay off the campaign trail for most of this week. He spent Tuesday at in Woodstock, Vermont, preparing for the three presidential debates that begin on October 3.
He was working with Ohio Republican Senator Rob Portman, who is playing the role of Obama in practice sessions, as well as top campaign aides Beth Myers, Stuart Stevens and Eric Fehrnstrom, among others.
(Additional reporting by Eric Johnson, Sam Youngman, and Margaret Chadbourn; Editing by Alistair Bell and Alden Bentley)