SPECIAL CORRESPONDENTWASHINGTON - US Democratic Party officials Tuesday threw their full weight behind President Barack Obama's policies, urging voters to give him a second four-year term as they opened their national convention with sharp criticism of Republican Party candidates Mitt Romney.America's First Lady Michelle Obama was the overwhelming star of the evening at the start of the three-day gathering in Charlotte, 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 President 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. The election will be held on November 6.Mrs Obama said her husband bases his presidential decisions on his belief of what's right for the United States, not political gain as she closed the first day of speeches at the convention, by painting a portrait of the man out of the public eye."For Barack, these issues are not political, they're personal," Obama said. "Because Barack knows what it means when a family struggles. He knows what it means to want something more for your kids and grandkids. Barack knows the American dream because he's lived it."Being president doesn't change who you are, she said. "No, it reveals who you are," she said.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 per cent 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, said"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 said 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 US manufacturing shrank at its sharpest clip in more than three years last month, while exports and hiring in the sector also slumped.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 countered  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.